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Digg Chapter 12. References ExplainedTable of Contents References in PHP are a means to access the same variable content by different names. They are not like C pointers; instead, they are symbol table aliases. Note that in PHP, variable name and variable content are different, so the same content can have different names. The most close analogy is with Unix filenames and files - variable names are directory entries, while variable contents is the file itself. References can be thought of as hardlinking in Unix filesystem. Code Examples / Notes » language.referencesiryoku
You should have in mind that php4 keep assigned variables "automagically" referenced until they are overwritten. So the variable copy is not executed on assignment, but on modification. Say you have this: $var1 = 5; $var2 = $var1; // In this point these two variables share the same memory location $var1 = 3; // Here $var1 and $var2 have they own memory locations with values 3 and 5 respectively Don't use references in function parameters to speed up aplications, because this is automatically done. I think that this should be in the manual, because it can lead to confusion. More about this here: http://www.zend.com/zend/art/ref-count.php jlaing
While trying to do object references with the special $this variable I found that this will not work: class foo { function bar() { ... $this =& $some_other_foo_obj; } } If you want to emulate this functionality you must iterate through the vars of the class and assign references like this: $vars = get_class_vars('foo'); foreach (array_keys($vars) as $field) { $this->$field =& $some_other_foo_obj->$field; } Now if you modify values within $this they will be modified within $some_other_foo_obj and vice versa. Hope that helps some people! p.s. developer at sirspot dot com's note about object references doesn't seem correct to me. $temp =& $object; $object =& $temp->getNext(); Does the same exact thing as: $object =& $object->getNext(); when you refernce $temp to $object all it does is make $temp an alias to the same memory as $object, so doing $temp->getNext(); and $object->getNext(); are calling the same function on the same object. Try it out if you don't believe me. developer
Since references are more like hardlinks than pointers, it is not possible to change a reference to an object by using that same reference. For example: The following WILL NOT WORK as expected and may even crash the PHP interpreter: $object =& $object->getNext(); However, by changing the previous statement to use a temporary reference, this WILL WORK: $temp =& $object; $object =& $temp->getNext(); ed
Responding to Slava Kudinov. The only reason why your script takes longer when you pass by reference is that you do not at all modify the array that your passing to your functions. If you do that the diffrences in execution time will be a lot smaller. In fact passing by reference will be faster if just by a little bit.
cesoid
Responding to post from nathan (who was responding to iryoku). It is important to note the difference between what php is doing from the programmer's point of view and what it is doing internally. The note that nathan refers to, about how (for example) $something = $this makes a "copy" of the current object, is talking about making a "copy" from the programmer's perspective. That is, for the programmer, for all practical purposes, $something is a copy, even if internally nothing has been copied yet. For example, changing the data in member $something->somethingVar will not change your current object's data (i.e. it will not change $this->somethingVar). What it does internally is a totally different story. I've tested "copying" an object which contains a 200,000 element array, it takes almost no time at all until you finally change something in one of the copies, because internally it only makes the copy when it becomes necessary. The original assignment takes less than a millisecond, but when I alter one of the copies, it takes something like a quarter of a second. But this only happens if I alter the 200,000 element array, if I alter a single integer of the object, it takes less than a microsecond again, so the interpretter seems to be smart enough to make copies of some of the objects variables and not others. The result is that when you change a function to pass by reference, it will only become more efficient if, inside the function, the passed variable is having its data altered, in which case passing by reference causes your code to alter the data of the original copy. If you are passing an object and calling a function in that object, that function may alter the object without you even knowing, which means that you should pass an object by reference as long as it is ok for the original copy to be effected by what you do with the object inside the function. I think the real moral of the story is this: 1) Pass by reference anything that should refer to and affect the original copy. 2) Pass not by reference things that will definitely not be altered in the function (for an object, it may be impossible to know whether it alters itself upon calling one of its functions). 3) If something needs to be altered inside a function without effecting the original copy, pass it not by reference, and pass the smallest practical part that needs to change, rather than passing, for example, a huge array of which one little integer will be altered. Or a shorter version: Only pass things by reference when you need to refer to the original copy! (And don't pass huge arrays or long strings when you need to change just a small part of them!) jw
Re-using variables which where references before, without unsetting them first, leads to unexpected behaviour. The following code: <?php $numbers = array(); for ($i = 1; $i < 4; $i++) { $numbers[] = null; $num = count($numbers); $index =& $numbers[$num ? $num - 1 : $num]; $index = $i; } foreach ($numbers as $index) { print "$index\n"; } ?> Does not produce: 1 2 3 But instead: 1 2 2 Applying unset($index) before re-using the variable fixes this and the expected list will be produced: 1 2 3 thenewparadigm
one very useful aspect for reference that i don't think i saw documented was the ability to skip a few steps with objects stored in objects. for example: assuming the object structure is correctly constructed (and mind you i haven't tried this in php, but it does work in most other high-end programming languages), instead of using this structure to get a variable/function //start $obj1 -> obj2 -> obj3 -> varX = 0; $obj1 -> obj2 -> obj3 -> varY = 0; $obj1 -> obj2 -> obj3 -> functionX(); $obj1 -> obj2 -> obj3 -> functionY(); //end you can use this method: //start $tempObj = & $obj1 -> obj2 -> obj3; $tempObj -> varX = 0; $tempObj -> varY = 0; $tempObj -> functionX(); $tempObj -> functionY(); //end note, if you want to use a shortcut variable to modify the original object you must include the ampersand (&) to reference the variable, otherwise if you used this line of code //start $tempObj = $obj1 -> obj2 -> obj3; //end any changes you make to $tempObj will not change the original object and may compromise the object structure, not to mention that it takes up extra memory. however, if you are just using the shortcut variable for read-only purposes, not using a reference wont cause any problems. another alternative in programming languages is the 'with' structure as seen below //start with($obj1 -> obj2 -> obj3) { varX = 0; varY = 0; functionX(); functionY(); } //end however, i don't expect this will work because as far as i've seen the 'with' structure is not supported in php. nathan
On the post that says php4 automagically makes references, this appears to *not* apply to objects: http://www.php.net/manual/en/language.references.whatdo.php "Note: Not using the & operator causes a copy of the object to be made. If you use $this in the class it will operate on the current instance of the class. The assignment without & will copy the instance (i.e. the object) and $this will operate on the copy, which is not always what is desired. Usually you want to have a single instance to work with, due to performance and memory consumption issues." maghiel
Note that: Call-time pass-by-reference has been deprecated - argument passed by value; If you would like to pass it by reference, modify the declaration of xxxxx. If you would like to enable call-time pass-by-reference, you can set allow_call_time_pass_reference to true in your INI file. However, future versions may not support this any longer hkmaly
It seems like PHP has problems with references, like that it can't work properly with circular references or free properly structure with more references. See http://bugs.php.net/?id=30053. I have big problem with this and I hope someone from PHP add proper warning with explanation IN manual, if they can't fix it. carlos
in the example below, you would get the same result if you change the function to something like: function test_ref(&$arr) { $time = time(); $size = sizeof($arr); // <--- this makes difference... for($n=0; $n<$size; $n++) { $x = 1; } echo "<br />The function using a reference took ".(time() - $time)." s"; } trucex
In response to Xor and Slava: I recommend you read up a bit more on the way PHP handles memory management. Take the following code for example: <?php $data = $_POST['lotsofdata']; $data2 = $data; $data3 = $data; $data4 = $data; $data5 = $data; ?> Assuming we post 10MB of data to this PHP file, what will PHP do with the memory? PHP uses a table of sorts that maps variable names to the data that variable refers to in memory. The $_POST superglobal will actually be the first instance of that data in the execution, so it will be the first variable referenced to that data in the memory. It will consume 10MB. Each $data var will simply point to the same data in memory. Until you change that data PHP will NOT duplicate it. Passing a variable by value does just what I did with each $data var. There is no significant overhead to assigning a new name to the same data. It is only when you modify the data passed to the function that it must allocate memory for the data. Passing a variable by reference will do essentially the same thing when you pass the data to the function, only modifying it will modify the data that is in the memory already versus copying it to a new location in memory. If for learning purposes you choose to disregard the obvious pointlessness in benchmarking the difference between these two methods of passing arguments, you will need to modify the data when it is passed to the function in order to obtain more accurate results. y007pig
In reply to pike at kw dot nl, '&' is only apply to PHP 4. PHP 5 changed the behavior and the object is defaultly passed by references and if you turn on E_STRICT, you will get a notice: Strict Standards: Assigning the return value of new by reference is deprecated in xxxx If you want to *copy* object in PHP 5, use object clone. sneskid
in addition to what 'jw at jwscripts dot com' wrote about unset; it can also be used to "detach" the variable alias so that it may work on a unique piece of memory again. here's an example <?php define('NL', "\r\n"); $v1 = 'shared'; $v2 = &$v1; $v3 = &$v2; $v4 = &$v3; echo 'before:'.NL; echo 'v1=' . $v1 . NL; echo 'v2=' . $v2 . NL; echo 'v3=' . $v3 . NL; echo 'v4=' . $v4 . NL; // detach messy $detach = $v1; unset($v1); $v1 = $detach; // detach pretty, but slower eval(detach('$v2')); $v1 .= '?'; $v2 .= ' no more'; $v3 .= ' sti'; $v4 .= 'll'; echo NL.'after:'.NL; echo 'v1=' . $v1 . NL; echo 'v2=' . $v2 . NL; echo 'v3=' . $v3 . NL; echo 'v4=' . $v4 . NL; function detach($v) { $e = '$detach = ' . $v . ';'; $e .= 'unset('.$v.');'; $e .= $v . ' = $detach;'; return $e; } ?> output { before: v1=shared v2=shared v3=shared v4=shared after: v1=shared? v2=shared no more v3=shared still v4=shared still } http://www.obdev.at/developers/articles/00002.html says there's no such thing as an "object reference" in PHP, but with detaching it becomes possible. Hopefully detach, or something like it, will become a language construct in the future. nslater
In addition to the note made by "Francis dot a at gmx dot net" you should not normally be using a function such as sizeof() or count() in a control structure such as FOR because the same value is being calculated repeatedly for each iteration. This can slow things down immensely, regardless of whether you pass by value or reference. It is generally much better to calculate the static values before the defining the looping control structure. Example: <?php $intSize = sizeof($arrData); for($i = 0; $i < $intSize; $n++) { // Do stuff } ?> slava kudinov
In addition to the "array vs &array" example below, php 5.1.2. It seems passing an array by reference slower than by value, consider: <?php function test_ref(&$arr) { $size = sizeof($arr); for($n=0; $n<$size; $n++) { $x = 1; } } function test_val($arr) { $size = sizeof($arr); for($n=0; $n<$size; $n++) { $x = 1; } } // fill array for($n=0; $n<2000; $n++) { $ar[] = "test".$n; } $time = microtime(); test_ref($ar); echo "<br />The function using a reference took ".(microtime() - $time)." s"; $time = microtime(); test_val($ar); echo "<br />The funktion using a value took: ".(microtime() - $time)." s"; echo "<br />Done"; ?> output: The function using a reference took 0.002694 s The funktion using a value took: 0.000847 s pike
if your object seems to "forget" assignments you make after instantiation, realize that in $foo = new Bar() the variable on the left hand is a *copy* of the variable on the right hand. As a result, & references made during instantiation may point to the righthandside version of Bar() and not to $foo. you'd better use $foo = & new Bar() xor
if given little - passing an array by reference slower than by value, but not if it is necessary to process much data. function test_ref(&$arr) { $size = sizeof($arr); for($n=0; $n<$size; $n++) { $x .= $arr[$n]; } return $x; } function test_val($arr) { $size = sizeof($arr); for($n=0; $n<$size; $n++) { $x .= $arr[$n]; } return $x; } // fill array for($n=0; $n<500000; $n++) { $ar[] = "test".$n; } $Bs = array_sum(explode(' ', microtime())); test_ref($ar); echo "<br />The function using a reference took ".(array_sum(explode(' ', microtime()))-$Bs); $Bs = array_sum(explode(' ', microtime())); test_val($ar); echo "<br />The funktion using a value took: ".(array_sum(explode(' ', microtime()))-$Bs); The function using a reference took 10.035583019257 The funktion using a value took: 10.531390190125 gunter dot sammet
I tried to create an array with n depth using a recursive function passing array references around. So far I haven't had much luck and I couldn't find anything on the web. So I ended up using eval() and it seems to work well: <?php foreach(array_keys($this->quantity_array) AS $key){ if($this->quantity_array[$key] > 0){ $combinations = explode('-', $key); $eval_string = '$eval_array'; foreach(array_keys($combinations) AS $key2){ $option_key_value = explode('_', $combinations[$key2]); $eval_string .= '['.$option_key_value[0].']['.$option_key_value[1].']'; } $eval_string .= ' = '.$this->quantity_array[$key].';'; eval($eval_string); } } ?> This produces an n dimensional array that will be available in the $eval_array variable. Hope it helps somebody! php
I found a very useful summary of how references work in PHP4 (and some of the common pitfalls) in this article: http://www.obdev.at/developers/articles/00002.html It deals with some subtle situations and I recommend it to anyone having difficulty with their references. grayson
I found a subtle feature of references that caused a bug in one of my PHP applications. In short, if an object passes one of its members to an external function that takes a reference as an argument, the external function can turn that member into a reference to an anonymous point in memory. Why is this a problem? Later, when you copy the object with $a = $b, the copy and the original share memory. Solution: If you want to have a function that uses references to modify a member of your object, your object should never pass the member to the function directly. It should first make a copy of the member. Then give that copy to the function. Then copy the new value of that copy in to your original object member. Below is some code that can reporoduce the this feature and demonstrate the workaround. function modify1 ( &$pointer_obj ){ $pointer_obj->property = 'Original Value'; } function modify2 ( &$pointer_obj ){ $newObj->property = 'Original Value'; $pointer_obj = $newObj; } class a { var $i; # an object with properties function corrupt1(){ modify1 ($this->i); } function doNotCorrupt1(){ $tmpi = $this->i; modify1 ($tmpi); $this->i = $tmpi; } function corrupt2(){ modify2 ($this->i); } function doNotCorrupt2(){ $tmpi = $this->i; modify2 ($tmpi); $this->i = $tmpi; } } $functions = array ('corrupt1', 'corrupt2', 'doNotCorrupt1', 'doNotCorrupt2'); foreach ($functions as $func){ $original = new a; ### Load some data in to the orginal with one of the four $functions $original->$func(); $copy = $original; $copy->i->property = "Changed after the copy was made."; echo "\n{$func}: \$original->i->property = '" . $original->i->property . "'"; } The script generates output: corrupt1: $original->i->property = 'Changed after the copy was made.' corrupt2: $original->i->property = 'Changed after the copy was made.' doNotCorrupt1: $original->i->property = 'Original Value' doNotCorrupt2: $original->i->property = 'Original Value' riseofthethorax
I don't see what the big fuss is, I've used pass by reference to modify dangling variables in tree structures serializing and deserializing structures to databases.. It seems the reason for this limitation is due to the 99% PHP Newbie syndrome and their lack of pointer experience.. Note the only difference between C and PHP references is there is no pointer arithmetic.. Whether it's an alias or a memory location with the address of another, it's basically the same thing.. If you wanted I could implement tree structures in a linear resizeable array. What am I supposed to do with array_walk(&$datastructure,'function'); ???? What am I not getting here.. What is the logic behind disabling it.. When did the PHP coders get all theoretical on us.. C is a great language because it keeps the idiots at bay, and allows you to shoot yourself in the foot if you really want to.. If I wanted to protect myself from bad code techniques I'd go write in python or lisp. Better yet why don't we send the 99% to school to get computer science degrees. francis dot a
I don't know if this is a bug (I'm using PHP 5.01) but you should be careful when using references on arrays. I had a for-loop that was incredibly slow and it took me some time to find out that most of the time was wasted with the function sizeof() at every loop, and even more time I spent finding out that this problem it must be somehow related to the fact, that I used a reference of the array. Take a look at the following example: function test_ref(&$arr) { $time = time(); for($n=0; $n<sizeof($arr); $n++) { $x = 1; } echo "<br />The function using a reference took ".(time() - $time)." s"; } function test_val($arr) { $time = time(); for($n=0; $n<sizeof($arr); $n++) { $x = 1; } echo "<br />The funktion using a value took: ".(time() - $time)." s"; } // fill array for($n=0; $n<2000; $n++) { $ar[] = "test".$n; } test_ref($ar); test_val($ar); echo "<br />Done"; When I tested it, the first function was done after 9 seconds, while the second (although the array must be copied) was done in not even one. The difference is inproportional smaller when the array size is reduced: When using 1000 loops the first function was running for 1 second, when using 4000 it wasn't even done after 30 Seconds. mramirez
For php programmers that come from pascal, in object pascal (delphi), variable references are used with the "absolute" keyword. PHP example: <?php global $myglobal; $myglobal = 5; function test() { global $myglobal; /*local*/ $mylocal =& $myglobal; echo "local: " . $mylocal . "\n"; echo "gloal: " . $myglobal . "\n"; } test(); ?> Pascal example: program dontcare; var myglobal: integer; procedure test; var mylocal ABSOLUTE myglobal; begin write("local: ", mylocal); write("global: ", myglobal); end; begin myglobal := 5; test; end. By the way, a "local" keyword in php for local variables, could be welcome :-) rlynch
And, further... <?php $intSize = sizeof($arrData); for($i = 0; $i < $intSize; $n++) { // Do stuff } ?> Can be shortened even further by using the often overlooked 'decrement until zero' concept. I don't much like decrementing to get a job done, but it can make a lot of sense when the evaluation-of-doneness is time-costly: <?php for($i = sizeof($arrData); $i-- > 0 ; ) { // Do stuff } ?> One less variable to toss on the heap. NOTE that rather inconveniently, the $i goes to and hits zero only in the case where it starts out positive. When it is zero to begin with, it will become -1. Bug or feature? I've used it as a feature as a quick test of whether the $arrData array was empty to begin with. If you don't plan on using $i after the loop for anything, it makes no difference. If you need to be doing something ascending-sequential though, then the temporary variable suggestions that others have made makes the most sense. Finally (and important) ... if the $arrData array is somehow being dynamically modified in size due to the //Do-Stuff routine underneath, then you may have little-to-no recourse except to use the sizeof() method/function in the loop. Or, keep up the temporary variable in the loop as you push and pop things into and off the array. (sigh... this is the notable failing of deeply linked internal data structures: to figure out the size of any linked list of arbitrarily composed elements, the entire array needs to be "walked" every time the sizeof() or count() method is used. But compared to the flexibility of associative arrays, the cost is mitigated by careful use.) GoatGuy henrik
A note for those of you that are using constructs like the following: return $this->$MyVarName in your objects. Consider the following: class Test { var $MyArray = array(); function add($var) { $this->$var[rand(1,100)] = rand(1,100); } function show($var) { echo "\nEcho from Test:\n"; print_r($this->$var); } } $test = new Test(); $test->show('MyArray'); $test->add('MyArray'); $test->add('MyArray'); $test->add('MyArray'); $test->show('MyArray'); will output Echo from Test: Array ( ) Fatal error: Cannot access empty property in /home/webroot/framework_devhost_dk/compiler/test2.php on line 5 For this to work properly you have to use a construct similar to this: class Test { var $MyArray = array(); function add($var) { $tmp =& $this->$var; //This is the trick... strange but true ;) $tmp[rand(1,100)] = rand(1,100); } function show($var) { echo "\nEcho from Test:\n"; print_r($this->$var); } } $test = new Test(); $test->show('MyArray'); $test->add('MyArray'); $test->add('MyArray'); $test->add('MyArray'); $test->show('MyArray'); Will output: Echo from Test: Array ( ) Echo from Test: Array ( [19] => 17 [53] => 57 [96] => 43 ) sneskid
(v5.1.4) One cool thing about var_dump is it shows which variables are references (when dumping arrays), symbolized by '∫' for int/null, and by '&' for boolean/double/string/array/object. I don't know why the difference in symmmmbolism. After playing around I found a better way to implement detaching (twas by accident). var_dump can show what's going on. <?php function &detach($v=null){return $v;} $A=array('x' => 123, 'y' => 321); $A['x'] = &$A['x']; var_dump($A); /* x became it's own reference... array(2) { ["x"]=> ∫(123) ["y"]=> int(321) }*/ $A['y']=&$A['x']; var_dump($A); /* now both are references array(2) { ["x"]=> ∫(123) ["y"]=> ∫(123) }*/ $z = 'hi'; $A['y']=&detach(&$z); var_dump($A); /* x is still a reference, y and z share array(2) { ["x"]=> ∫(123) ["y"]=> &string(2) "hi" }*/ $A['x'] = $A['x']; $A['y']=&detach(); var_dump($A,$z); /* x returned to normal, y is on its own, z is still "hi" array(2) { ["x"]=> int(123) ["y"]=> NULL }*/ ?> For detach to work you need to use '&' in the function declaration, and every time you call it. Use this when you know a variable is a reference, and you want to assign a new value without effecting other vars referencing that piece of memory. You can initialize it with a new constant value, or variable, or new reference all in once step. eduardofleury
<?php $foo = 'Hello'; $bar = 'World'; print $foo . " " . $bar;// Hello World $foo = &$bar; $bar = 'Hello My World'; print $foo;// Hello My World print $bar;// Hello My World ?> rmaj
<? $a = array("a"=>"111", "b"=>"222", "c"=>"333"); foreach ($a as $ix => $val) { $ref = &$a[$ix]; $ref = $val . '_changed'; } foreach ($a as $ix => $val) echo "$val "; // 111_changed 222_changed 333_changed ?> is a simply way to change elements of array witohut retyping $array_name[$index] all the time |
