declare

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(PHP 4, PHP 5)

declare 结构用来设定一段代码的执行指令。declare 的语法和其它流程控制结构相似：

declare (directive)
    statement

directive 部分允许设定 declare 代码段的行为。目前只认识两个指令：ticks（更多信息见下面 ticks 指令）以及 encoding（更多信息见下面 encoding 指令）。

Note: encoding 是 PHP 5.3.0 新增指令。

declare 代码段中的 statement 部分将被执行——怎样执行以及执行中有什么副作用出现取决于 directive 中设定的指令。

declare 结构也可用于全局范围，影响到其后的所有代码（但如果有 declare 结构的文件被其它文件包含，则对包含它的父文件不起作用）。

  <?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

Tick（时钟周期）是一个在 declare 代码段中解释器每执行 N 条可计时的低级语句就会发生的事件。 N 的值是在 declare 中的 directive 部分用 ticks= N 来指定的。

不是所有语句都可计时。通常条件表达式和参数表达式都不可计时。

在每个 tick 中出现的事件是由 register_tick_function() 来指定的。更多细节见下面的例子。注意每个 tick 中可以出现多个事件。

Example #1 Tick 的用法示例

  <?php

 declare( ticks = 1 );

 // A function called on each tick event
 function  tick_handler ()
{
    echo  "tick_handler() called\n" ;
}

 register_tick_function ( 'tick_handler' );

 $a  =  1 ;

if ( $a  >  0 ) {
     $a  +=  2 ;
    print( $a );
}

 ?>

Example #2 Ticks 的用法示例

  <?php

 function  tick_handler ()
{
  echo  "tick_handler() called\n" ;
}

 $a  =  1 ;
 tick_handler ();

if ( $a  >  0 ) {
     $a  +=  2 ;
     tick_handler ();
    print( $a );
     tick_handler ();
}
 tick_handler ();

 ?>

参见 register_tick_function() 和 unregister_tick_function() 。

Encoding

可以用 encoding 指令来对每段脚本指定其编码方式。

Example #3 对脚本指定编码方式

  <?php
 declare( encoding = 'ISO-8859-1' );
 // code here
 ?>

Caution

当和命名空间结合起来时 declare 的唯一合法语法是 declare(encoding='...');，其中 ... 是编码的值。而 declare(encoding='...') {} 将在与命名空间结合时产生解析错误。

在 PHP 5.3 中除非在编译时指定了 --enable-zend-multibyte，否则 declare 中的 encoding 值会被忽略。

注意除非用 phpinfo() ，否则 PHP 不会显示出是否在编译时指定了 --enable-zend-multibyte。

参见 zend.script_encoding。

用户评论:

[#1] danekilian at wp dot pl [2015-05-02 16:13:30]

We can specify different encoding to different blocks:

declare(encoding=ENCODING_VALUE) {
//for a block
}

found this here:
http://code2care.org/tutorials/php/PHP-Declare-ticks-and-tick-functions-Statements-Tutorial.php

[#2] Kubo2 [2015-04-06 17:13:56]

Note that in PHP 7 <?php declare(encoding='...'); ?> throws an E_WARNING if Zend Multibyte is turned off.

[#3] php at niekbosch dot nl [2014-02-18 16:17:54]

Basically 'declare( encoding = .... );' overrides the zend.script_encoding configuration option (as set in php.ini). However, keep in mind that:

* the file encoding must be compatible (at least in the ASCII range of characters) to the zend.script_encoding setting. If you set 'zend.script_encoding' to UTF-8 and save the file in UTF-16, PHP will not be able to interpret the file, let alone the declare statement. As long as you use ASCII compatible encodings (i.e. ISO-8859-1(5), UTF-8 etc) for both the file encoding as the zend.script_encoding, you should be fine. (However, I have not experimented with adding non-ascii characters in comments above the declare statement).

* PHP string literals are converted from your source code encoding (either set with the declare statement or else according to zend.script_encoding) to the mbstring.internal_encoding as set in your php.ini (even if you change the setting using mb_internal_encoding). As an example:

php.ini:
mbstring.internal_encoding = UTF-8

test.php:
<?php declare(encoding = 'ISO-8859-15'); mb_internal_encoding( 'ISO-8859-15' ); echo 'a?a?' . "\n"; ?>

This will still output the string UTF-8 encoded; in a terminal/browser with encoding 'ISO-8859-15' the string will look (something) like this: a??a??

[#4] nospam dot narf at bofh dot bg [2014-02-13 14:43:48]

This manual doesn't say what "script's encoding" means and how declaring it affects its behavior.

Of course declare(encoding='foo') would specify the encoding - that's self-explanatory and not helpful.

[#5] php dot net at e-z dot name [2013-10-17 06:34:42]

you can register multiple tick functions:

<?PHP function a() { echo "a\n"; } function b() { echo "b\n"; } register_tick_function('a'); register_tick_function('b'); register_tick_function('b'); register_tick_function('b'); ?>

will output on every tick:
a
b
b
b

[#6] sawyerrken at gmail dot com [2013-08-09 06:31:37]

In the following example:

<?php function handler(){ print "hello "; } register_tick_function("handler"); declare(ticks = 1){ $b = 2; } //closing curly bracket tickable ?>

"Hello" will be displayed twice because the closing curly bracket is also tickable.

One may wonder why the opening curly bracket is not tickable if the closing is tickable. This is because the instruction for PHP to start ticking is given by the opening curly bracket so the ticking starts immediately after it.

[#7] Kamil Pawelkiewicz [2012-10-10 18:56:52]

I've created memory usage monitor class using tick event.

The result is returned in a fancy graph using GD library.

You can get the source, readme and example script at:
https://github.com/kampaw/profiler

Usage is very simple:

<?php require('profiler.php'); $profiler = new profiler; declare(ticks = 1000); // monitor started // insert your code here $profiler->chart(); ?>

[#8] ramamneh at gmail dot com [2010-12-19 11:15:47]

check loaded server connection

<?php $connection = false; function checkConnection( $connectionWaitingTime = 3 ) { // check connection & time global $time,$connection; if( ($t = (time() - $time)) >= $waitingTime && !$connection){ echo (" Server not responding for $t seconds !! "); die("Connection aborted"); } } register_tick_function("checkConnection"); $time = time(); declare (ticks=1) { while( true ){ // connecting to loaded server } $connection = true ; } ?>

[#9] Anonymous [2010-09-27 14:10:48]

It's amazing how many people didn't grasp the concept here. Note the wording in the documentation. It states that the tick handler is called every n native execution cycles. That means native instructions, not including system calls (i'm guessing). This can give you a very good idea if you need to optimize a particular part of your script, since you can measure quite effectively how many native instructions are in your actual code.

A good profiler would take that into account, and force you, the developer, to include calls to the profiler as you're entering and leaving every function. That way you'd be able to keep an eye on how many cycles it took each function to complete. Independent of time.

That is extremely powerful, and not to be underestimated. A good solution would allow aggregate stats, so the total time in a function would be counted, including inside called functions.

[#10] markandrewslade at dontspamemeat dot gmail [2009-02-13 09:06:02]

Note that the two methods for calling declare are not identical.

Method 1:

<?php // Print "tick" with a timestamp and optional suffix. function do_tick($str = '') { list($sec, $usec) = explode(' ', microtime()); printf("[%.4f] Tick.%s\n", $sec + $usec, $str); } register_tick_function('do_tick'); // Tick once before declaring so we have a point of reference. do_tick('--start--'); // Method 1 declare(ticks=1); while(1) sleep(1); ?>

Method 2:
<?php // Print "tick" with a timestamp and optional suffix. function do_tick($str = '') { list($sec, $usec) = explode(' ', microtime()); printf("[%.4f] Tick.%s\n", $sec + $usec, $str); } register_tick_function('do_tick'); // Tick once before declaring so we have a point of reference. do_tick('--start--'); // Method 2 declare(ticks=1) { while(1) sleep(1); } ?>

Notice that when using {} after declare, do_tick wasn't auto-called until about 1 second after we entered the declare {} block. However when not using the {}, do_tick was auto-called not once but twice immediately after calling declare();.

I'm assuming this is due to how PHP handles ticking internally. That is, declare() without the {} seems to trigger more low-level instructions which in turn fires tick a few times (if ticks=1) in the act of declaring.

[#11] anotheruser at example dot com [2008-08-23 16:50:11]

Code evaluation script which uses debug_backtrace() to get execution time in ns, relative current line number, function, file, and calling function info on each tick, and shove it all in $script_stats array. See debug_backtrace manual to customize what info is collected.

Warning: this will exhaust allowed memory very easily, so adjust tick counter according to the size of your code. Also, array_key_exists checking on debug_backtrace arrays is removed here only to keep this example simple, but should be added to avoid a large number of resulting PHP Notice errors.

<?php $script_stats = array(); $time = microtime(true); function track_stats(){ global $script_stats,$time; $trace = debug_backtrace(); $exe_time = (microtime(true) - $time) * 1000; $func_args = implode(", ",$trace[1]["args"]); $script_stats[] = array( "current_time" => microtime(true), "memory" => memory_get_usage(true), "file" => $trace[1]["file"].': '.$trace[1]["line"], "function" => $trace[1]["function"].'('.$func_args.')', "called_by" => $trace[2]["function"].' in '.$trace[2]["file"].': '.$trace[2]["line"], "ns" => $exe_time ); $time = microtime(true); } declare(ticks = 1); register_tick_function("track_stats"); // the rest of your project code // output $script_stats into a html table or something ?>

[#12] zabmilenko at charter dot net [2008-01-08 01:49:59]

If you misspell the directive, you won't get any error or warning. The declare block will simply act as a nest for statements:

<?php declare(tocks="four hundred") { // Has no affect on code and produces // no error or warning. } ?>

Tested in php 5.2.5 on XPsp2

[#13] rosen_ivanov at abv dot bg [2006-08-28 06:06:33]

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); 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 :)

[#14] aeolianmeson at NOSPAM dot blitzeclipse dot com [2006-05-30 00:06:16]

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:

<?php 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.

[#15] warhog at warhog dot net [2005-12-18 12:39:30]

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 } ?>

[#16] chris-at-free-source.com [2005-02-28 12:16:34]

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.

[#17] fok at nho dot com dot br [2003-07-07 18:45:59]

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 " \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. =--

[#18] daniel@swn [2003-02-01 11:56:13]

<?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 ".time()." \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); }

[#19] rob_spamsux at rauchmedien dot ihatespam dot com [2002-03-19 02:45:14]

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.