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PHP : Language Reference : Control Structures : declare

declare

The declare construct is used to set execution directives for a block of code. The syntax of declare is similar to the syntax of other flow control constructs:

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declare (directive)
   statement

The directive section allows the behavior of the declare block to be set. Currently only one directive is recognized: the ticks directive. (See below for more information on the ticks directive)

The statement part of the declare block will be executed -- how it is executed and what side effects occur during execution may depend on the directive set in the directive block.

The declare construct can also be used in the global scope, affecting all code following it.

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<?php
// these are the same:

// you can use this:
declare(ticks=1) {
   // entire script here
}

// or you can use this:
declare(ticks=1);
// entire script here
?>

Ticks

A tick is an event that occurs for every N low-level statements executed by the parser within the declare block. The value for N is specified using ticks=N within the declare blocks's directive section.

The event(s) that occur on each tick are specified using the register_tick_function(). See the example below for more details. Note that more than one event can occur for each tick.

Example 7.3. Profile a section of PHP code

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<?php
// A function that records the time when it is called
function profile($dump = FALSE)
{
   static $profile;

   // Return the times stored in profile, then erase it
   if ($dump) {
       $temp = $profile;
       unset($profile);
       return $temp;
   }

   $profile[] = microtime();
}

// Set up a tick handler
register_tick_function("profile");

// Initialize the function before the declare block
profile();

// Run a block of code, throw a tick every 2nd statement
declare(ticks=2) {
   for ($x = 1; $x < 50; ++$x) {
       echo similar_text(md5($x), md5($x*$x)), "<br />;";
   }
}

// Display the data stored in the profiler
print_r(profile(TRUE));
?>


The example profiles the PHP code within the 'declare' block, recording the time at which every second low-level statement in the block was executed. This information can then be used to find the slow areas within particular segments of code. This process can be performed using other methods: using ticks is more convenient and easier to implement.

Ticks are well suited for debugging, implementing simple multitasking, background I/O and many other tasks.

See also register_tick_function() and unregister_tick_function().

Related Examples ( Source code ) » control_structures.declare

Examples ( Source code ) » Get the filename from the descriptor BFILE

Code Examples / Notes » control_structures.declare

fok
This is a very simple example using ticks to execute a external script to show rx/tx data from the server
<?php
function traf(){
 passthru( './traf.sh' );
 echo "<br />\n";
 flush(); // keeps it flowing to the browser...
 sleep( 1 );
}
register_tick_function( "traf" );
declare( ticks=1 ){
 while( true ){}   // to keep it running...
}
?>
contents of traf.sh:
# Shows TX/RX for eth0 over 1sec
#!/bin/bash
TX1=`cat /proc/net/dev | grep "eth0" | cut -d: -f2 | awk '{print $9}'`
RX1=`cat /proc/net/dev | grep "eth0" | cut -d: -f2 | awk '{print $1}'`
sleep 1
TX2=`cat /proc/net/dev | grep "eth0" | cut -d: -f2 | awk '{print $9}'`
RX2=`cat /proc/net/dev | grep "eth0" | cut -d: -f2 | awk '{print $1}'`
echo -e "TX: $[ $TX2 - $TX1 ] bytes/s \t RX: $[ $RX2 - $RX1 ] bytes/s"
#--= the end. =--

aeolianmeson
The scope of the declare() call if used without a block is a little unpredictable, in my experience. It appears that if placed in a method or function, it may not apply to the calls that ensue, like the following:
function a()
{
  declare(ticks=2);
  b();
}
function b()
{
  // The declare may not apply here, sometimes.
}
So, if all of a sudden the signals are getting ignored, check this. At the risk of losing the ability to make a mathematical science out of placing a number of activities at varying durations of ticks like many people have chosen to do, I've found it simple to just put this at the top of the code, and just make it global.

xxoes
If i use ticks i must declare all functions before i call the function.
example:
Dosn't work
<?php
function ticks() {
  echo "tick";
}
register_tick_function("ticks");
declare (ticks=1) 1;
echo "";
echo "";
foo(); // Call to undefined function.
function foo() {
  echo "foo";
}
?>
Work
<?php
function ticks() {
  echo "tick";
}
register_tick_function("ticks");
//declare (ticks=1) 1;
echo "";
echo "";
foo();
function foo() {
  echo "foo";
}
?>
win2k : PHP 4.3.0 (cgi-fcgi)

rob_spamsux
Here is an example of multi-tasking / multi-threading:
<?php
# declare functions
function a() {
 echo "a";
}
function b() {
 echo "b";
}
# register tick functions
register_tick_function ("a");
register_tick_function ("b");
# make the tick functions run
declare (ticks=1);
# that's all there is to it.
?>
Notes:
This will make functions a and b run once each at the same time.
If you try:
declare (ticks=1) {
 1;
}
They will run twice each. That is because it seems to be an undocumented fact that there is always an extra tick.
Therefore:
declare (ticks=2) {
 1;
}
Will cause them to run once.

zsh
declare does also support an alternative syntax (see control-structures.alternative-syntax):
<?php
declare (ticks=1):
// Funky shizzle
enddeclare;
?>
As you can see, enddeclare is (highlighted as) a keyword.

rob_spamsux
Correction to above note:
Apparently, the end brace '}' at the end of the statement causes a tick.
So using
------------
declare (ticks=1) echo "1 tick after this prints";
------------
gives the expected behavior of causing 1 tick.
Note: the tick is issued after the statement executes.
Also, after playing around with this, I found that it is not really the multi-tasking I had expected. It behaves the same as simply calling the functions. I.e. each function must finish before passing the baton to the next function. They do not run in parallel.
It also seems that they always run in the order in which they were registered.
So,
<?php
------------
# register tick functions
register_tick_function ("a");
register_tick_function ("b");
# make the tick functions run
declare (ticks=1);
?>
------------
is equivalent to
------------
a();
b();
------------
It is simply a convenient way to have functions called periodically while some other code is being executed. I.e. you could use it to periodically check the status of something and then exit the script or do something else based on the status.

warhog
as i read about ticks the first time i thought "wtf, useless crap" - but then i discovered some usefull application...
you can declare a tick-function which checks each n executions of your script whether the connection is still alive or not, very usefull for some kind of scripts to decrease serverload
<?php
function check_connection()
{ if (connection_aborted())
  { // do something here, e.g. close database connections
     // (or  use a shutdown function for this
     exit; }
}
register_tick_function("connection");
declare (ticks=20)
{
 // put your PHP-Script here
 // you may increase/decrease the number of ticks
}
?>

rosen_ivanov
As Chris already noted, ticks doesn't make your script multi-threaded, but they are still great. I use them mainly for profiling - for example, placing the following at the very beginning of the script allows you to monitor its memory usage:
<?php
function profiler($return=false) {
   static $m=0;
   if ($return) return "$m bytes";
   if (($mem=memory_get_usage())>$m) $m = $mem;
}
register_tick_function('profiler');
declare(ticks=1);
/*
Your code here
*/
echo profiler(true);
?>
This approach is more accurate than calling memory_get_usage only in the end of the script. It has some performance overhead though :)

chris-at-free-source.com
Also note that PHP is run in a single thread and so everything it does will be one line of code at a time.  I'm not aware of any true threading support in PHP, the closest you can get is to fork.
so, declare tick doens't "multi-thread" at all, it is simply is a way to automaticaly call a function every n-lines of code.

daniel@swn
<?php
ob_end_clean();
ob_implicit_flush(1);
function a() {
for($i=0;$i<=100000;$i++) { }
echo "function a() ";
}
function b() {
for($i=0;$i<=100000;$i++) { }
echo "function b() ";
}
register_tick_function ("a");
register_tick_function ("b");
declare (ticks=4)
{
while(true)
{
sleep(1);
echo "\n
<b>".time()."</b>
\n";;
}
}
?>
You will see that a() and b() are slowing down this process. They are in fact not executed every second as expected. So this function is not a real alternative for multithreading using some slow functions..there is no difference to this way: while (true) { a(); b(); sleep(1); }

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