`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:

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.

<?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 16-3. Profile a section of PHP code

<?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.