foreach

PHP 4 introduced a foreach construct, much like Perl and some other languages. This simply gives an easy way to iterate over arrays. foreach works only on arrays, and will issue an error when you try to use it on a variable with a different data type or an uninitialized variable. There are two syntaxes; the second is a minor but useful extension of the first:

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foreach (array_expression as $value)

    statement

foreach (array_expression as $key => $value)

    statement

The first form loops over the array given by array_expression. On each loop, the value of the current element is assigned to $value and the internal array pointer is advanced by one (so on the next loop, you'll be looking at the next element).

The second form does the same thing, except that the current element's key will be assigned to the variable $key on each loop.

As of PHP 5, it is possible to iterate objects too.

Note:

When foreach first starts executing, the internal array pointer is automatically reset to the first element of the array. This means that you do not need to call reset() before a foreach loop.

Note:

Unless the array is referenced, foreach operates on a copy of the specified array and not the array itself. Therefore, the array pointer is not modified as with the each() construct, and changes to the array element returned are not reflected in the original array. However, the internal pointer of the original array is advanced with the processing of the array. Assuming the foreach loop runs to completion, the array's internal pointer will be at the end of the array.

As of PHP 5, you can easily modify array's elements by preceding $value with &. This will assign reference instead of copying the value.

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<?php

$arr = array(1, 2, 3, 4);

foreach ($arr as &$value) {

    $value = $value * 2;

}
// $arr is now array(2, 4, 6, 8)
unset($value); // break the reference with the last element
?>

This is possible only if iterated array can be referenced (i.e. is variable).

Warning:

Reference of a $value and the last array element remain even after the foreach loop. It is recommended to destroy it by unset().

Note:

foreach does not support the ability to suppress error messages using '@'.

You may have noticed that the following are functionally identical:

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<?php

$arr = array("one", "two", "three");
reset($arr);

while (list(, $value) = each($arr)) {

    echo "Value: $value<br />\n";

}


foreach ($arr as $value) {

    echo "Value: $value<br />\n";

}
?>

The following are also functionally identical:

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<?php

$arr = array("one", "two", "three");
reset($arr);

while (list($key, $value) = each($arr)) {

    echo "Key: $key; Value: $value<br />\n";

}


foreach ($arr as $key => $value) {

    echo "Key: $key; Value: $value<br />\n";

}
?>

Some more examples to demonstrate usages:

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<?php
/* foreach example 1: value only */

$a = array(1, 2, 3, 17);


foreach ($a as $v) {

   echo "Current value of \$a: $v.\n";

}

/* foreach example 2: value (with key printed for illustration) */

$a = array(1, 2, 3, 17);

$i = 0; /* for illustrative purposes only */

foreach ($a as $v) {

    echo "\$a[$i] => $v.\n";

    $i++;

}

/* foreach example 3: key and value */

$a = array(

    "one" => 1,

    "two" => 2,

    "three" => 3,

    "seventeen" => 17
);


foreach ($a as $k => $v) {

    echo "\$a[$k] => $v.\n";

}

/* foreach example 4: multi-dimensional arrays */
$a = array();
$a[0][0] = "a";
$a[0][1] = "b";
$a[1][0] = "y";
$a[1][1] = "z";


foreach ($a as $v1) {

    foreach ($v1 as $v2) {

        echo "$v2\n";

    }

}

/* foreach example 5: dynamic arrays */

foreach (array(1, 2, 3, 4, 5) as $v) {

    echo "$v\n";

}
?>

Related Examples ( Source code ) » control_structures.foreach

Code Examples / Notes » control_structures.foreach

Using the much-maligned ability of foreach to work on references rather than copies, it becomes possible to recurse indefinitely into deep arrays without the need to know beforehand how deep they go. Consider the following example (which I use to sanitize input): <?php // Sanitize as a function to allow recursing; original array passed by reference function sanitize(&$array) { foreach ($array as &$data) { if (!is_array($data)) { // If it's not an array, clean it $data = '\\' . $data; // addslashes(), mysql_real_escape_string() or whatever you wish to use, this is merely a simple example } else { // If it IS an array, call the function on it sanitize($data); } } } // Test case $test = array( array( array( 0, 1, 2 ), 3, 4 ), 5, 6 ); // Output sanitize($test); print_r($test); /* Output: Array ( [0] => Array ( [0] => Array ( [0] => \0 [1] => \1 [2] => \2 ) [1] => \3 [2] => \4 ) [1] => \5 [2] => \6 ) */ ?> When first called on $test, it passes a reference to the original $test array to the sanitize function, which begins to iterate through its contents with the immediate foreach loop. Each value that is not in itself an array gets sanitized as normal, and as both the foreach loop and the function itself are acting by reference, all changes happen directly to the contents of the superglobal rather than to copies. If the value given IS an array, it then calls the same function on it. The key here is that both the function and the foreach loop work by reference, meaning that you can call the calling function while in the foreach loop and all changes are still applied to the original array, without corruption of the array pointer, as it remains intact for each level of the array. When the end of an array is reached, the function simply returns; if it was a deeper-level array, the parent function (and parent foreach loop) continue as they were; if the top-level loop ends, then the function returns to the main code, having acted on the entire array. As everything operates within the scope of the sanitize function, you even avoid the danger of leaving the last reference set, as $data is not available outside the scope of the function in which any particular loop operates. While this might sound complicated at first, the result is that by passing to foreach by reference, true indefinite recursion is possible.

Using PHP5's foreach "as reference" can bite you! Three guys in my office spent about a day chasing this one's tail, that was causing aberrant behavior in the values of elements of an array. It turns out to be a consequence of the nature of references, generally. If you create a reference to a variable, all names for that variable (including the original) BECOME REFERENCES. To paraphrase "The Highlander," if you want a name to OWN a piece of data, "there can be only one." To illustrate this point, consider the following code: <?php $f = array( 0 => array('value' => 'three'), 1 => array('value' => 'three') ); foreach ( $f as $k => &$v ) { $v['value'] = 'one'; } $a = $f; $b = $f; $b[0]['value'] = 'two'; $b[1]['value'] = 'two'; var_dump($a, $b); ?> Upon execution, you will find that, although you would expect $a to contain two arrays with 'value' of 'one', $a and $b are identical -- i.e., the changes of []['value'] to 'two' have happened in both arrays. But, upon further examination of the var_dumps, you will see that both sets' elements [0] and [1] are preceded with "&": they are references! The easy solution to this problem turns out to be: unset the foreach "as-reference" variable ($v) at the bottom of your foreach loop. This allows the original variable (or array member) to resume ownership of the value and dissolves its "reference-ness".

This is a summary of bug report #26396 having status "wont fix", so the following is not a bug (report), but may need extra highlighting so the novice programmer (like me) can make sense of the second note given above. Note: Also note that foreach operates on a copy of the specified array and not the array itself. Therefore, the array pointer is not modified as with the each() construct, and changes to the array element returned are not reflected in the original array. However, the internal pointer of the original array is advanced with the processing of the array. Assuming the foreach loop runs to completion, the array's internal pointer will be at the end of the array. <? $myArray = array("a", "b"); foreach($myArray as $anElement) { foreach($myArray as $anotherElement) { echo $anotherElement; } } ?> results in "abab", as each foreach works on a copy of $myArray. However: <? $myArray = array("a", "b"); function b() { global $myArray; foreach($myArray as $anotherElement) { echo $anotherElement; } } function a() { global $myArray; foreach($myArray as $anElement) { b(); } } a(); ?> results in "ab", ie. both foreach work on the same instance of $myArray because it is referenced as a global variable. Nevertheless, to the casual observer both variants seem equivalent and therefore should produce the same output.

There is a really really big pitfall to watch out for if you are using "foreach" and references. Recall this example: <? $a = "Hello"; $b =& $a; // $b now refers to "Hello" $b = "Goodbye"; // BOTH $a and $b now refer to "Goodbye" ?> This also applies to the loop variable in a foreach construct. This can be a problem if the loop variable has already been defined as a reference to something else. For example: <? // Create some objects and store them in an array $my_objects = array(); for($a = 0; $a < $num_objects; $a++) { $obj =& new MyObject(); $obj->doSomething(); $my_objects[] =& $obj; } // later on in the same function... foreach($my_objects as $obj) { // Note that we are trying to re-use $obj as the loop variable $obj->doSomethingElse(); } ?> When the "for" loop exits, $obj is a reference to the last MyObject that was created, which is also the last element in the "my_objects" array. On every iteration, the foreach loop will do the equivalent of: <? $obj = $my_objects[$internal_counter++]; ?> $obj will now refer to the appropriate element in the array. But recall the reference example at the top. Because $obj was already defined as a reference, any assignment to $obj will overwrite what $obj was referring to. So in other words, on every foreach loop iteration, the last element in the array will be overwritten with the current array element. To avoid this problem, either use a differently named loop variable, or call "unset()" on the loop variable before you begin the foreach(). It would be more intuitive PHP unset() the loop variable before a foreach began, maybe they'll put that in a later version.

One great use of foreach is for recursive function calls. It can handle things that would otherwise need to be hand-coded down to the desired level, which can make for hard-to-follow nesting. <? /* May you never be forced to deal with a structure like this in real life... */ $items = array( -1, 0, array( array(1,2,3,4), array(5,6,7,8), array(9,10,11,12) ), array( array("a", "b", "c"), array("d", "e", "f"), array("g", "h", "i") ) ); print_array($items, 0); function print_array($arr, $level) { $indent = str_repeat(" ", $level * 4); foreach ($arr as $item) { if (is_array($item)) { print $indent . "Item is an array... \n"; print_array($item, $level + 1); } else print $indent . "Value at current index: " . $item . " \n"; } } ?> This will output: Value at current index: -1 Value at current index: 0 Item is an array... Item is an array... Value at current index: 1 Value at current index: 2 Value at current index: 3 Value at current index: 4 Item is an array... Value at current index: 5 Value at current index: 6 Value at current index: 7 Value at current index: 8 Item is an array... Value at current index: 9 Value at current index: 10 Value at current index: 11 Value at current index: 12 Item is an array... Item is an array... Value at current index: a Value at current index: b Value at current index: c Item is an array... Value at current index: d Value at current index: e Value at current index: f Item is an array... Value at current index: g Value at current index: h Value at current index: i And it could recurse as far as you want it to go.

I wrote this code to add each post from a user to every users' txt file. But it only adds the message to the last user in users.txt. The reason of that is...(questmark) <?php session_start(); header("Cach-control:private"); $name=$_SESSION['name']; $nameframe=$name.".txt"; $message=$_POST['message']; $wierdchars = array("\'", "\"", "\\", ":-)", ":-D", ":-p", ":-(", "=p", ">:0", ":-[", ":-/", ":-\\", ":-X", ":-?", "B-)"); $newchars = array (stripslashes("\'"), stripslashes("\""), "\\", "<img src='smile.gif'>", "<img src='opensmile.gif'>", "<img src='tounge.gif'>", "<img src='sad.gif'>", "<img src='tounge.gif'>", "<img src='yelling.gif'>", "<img src='embarrased.gif'>", "<img src='sosoleft.gif'>", "<img src='sosoright.gif'>", "<img src='quiet.gif'>", "<img src='confused.gif'>", "<img src='cool.gif'>"); $newmessage=str_replace($wierdchars, $newchars, $message); $fontcolor=$_POST['fontcolor']; $fontsize=$_POST['fontsize']; $fontface=$_POST['fontface']; $users=file("users.txt"); foreach($users as $user) { $nameframed=$user.".txt"; $thefile=file_get_contents($nameframed); $file=fopen($nameframed, "w+"); $body=$name."|".$newmessage."\n"; $body2=$body.$thefile; $write=fwrite($file,$body2); fclose($file); } echo "<html><head><title>Adding info...</title>"; echo "<script>window.location='frame.php';</script>"; echo "</head>"; echo "<!--Removes ads form page</head><body>"; ?>

foreach

Related Examples ( Source code ) » control_structures.foreach

Code Examples / Notes » control_structures.foreach

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