What is declare in php?

[PHP 4, PHP 5, PHP 7, PHP 8]

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 three directives are recognized: the ticks directive [See below for more information on the ticks directive], the encoding directive [See below for more information on the encoding directive] and the strict_types directive [See for more information the strict typing section on the type declarations page]

As directives are handled as the file is being compiled, only literals may be given as directive values. Variables and constants cannot be used. To illustrate:

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 [however if the file with declare was included then it does not affect the parent file].

Ticks

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

Not all statements are tickable. Typically, condition expressions and argument expressions are not tickable.

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 #1 Tick usage example

See also register_tick_function[] and unregister_tick_function[].

Encoding

A script's encoding can be specified per-script using the encoding directive.

Example #2 Declaring an encoding for the script.

Caution

When combined with namespaces, the only legal syntax for declare is declare[encoding='...']; where ... is the encoding value. declare[encoding='...'] {} will result in a parse error when combined with namespaces.

See also zend.script_encoding.

Anonymous ¶

11 years ago

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.

Kubo2 ¶

7 years ago

Note that in PHP 7 throws an E_WARNING if Zend Multibyte is turned off.

digitalaudiorock at gmail dot com ¶

3 years ago

A few important things to note for anyone using this in conjunction with signal handlers:

If anyone is trying to optionally use either pcntl_async_signals[] when available [PHP >= 7.1] or ticks for older versions, this is not possible...at least not in a way that does NOT enable ticks for newer PHP versions. This is because there is simply no way to conditionally declare ticks. For example, the following will "work" but not in the way you might expect:



While signal handlers will work with this for old and new version, ticks WILL be enabled even in the case where pcntl_async_signals exists, simply because the declare statement exists. So the above is functionally equivalent to:



Another thing to be aware of is that the scoping of this declaration changed a bit from PHP 5.6 to 7.x...actually it was corrected apparently as noted here:

//php.net/manual/en/function.register-tick-function.php#121204

This can cause some very confusing behavior. One example is with the pear/System_Daemon module. With PHP 5.6 that will work with a SIGTERM handler even if the script using it doesn't itself use declare[ticks=1], but does not work in PHP 7 unless the script itself has the declaration. Not only does the handler not get called, but the signal does nothing at all, and the script doesn't exit.

A side note regarding ticks that's annoyed me for some time: As if there wasn't enough confusion around all this, the Internet is full of false rumors that ticks were deprecated and are being removed, and I believe they all started here:

//www.hackingwithphp.com/4/21/0/the-declare-function-and-ticks

Despite a very obscure author's note at the very end of the page saying he got that wrong [that even I just noticed], the first very prominent sentence of the article still says this, and that page is near the top of any Google search.

sawyerrken at gmail dot com ¶

9 years ago

In the following example:



"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.

digitalaudiorock at gmail dot com ¶

3 years ago

Regarding my previous comment as to the change in scope of declare[ticks=1] between 5.6 and 7.x, I intended to mention another example of the affect this can have on signal handlers:

If your script uses declare[ticks=1] and assigns handlers, in 5.6 signals will get caught and call the handler even when the code that is running is in an included file [where the included file doesn't have the declaration]. However in 7.x the signal wouldn't get caught until the code returns to the main script.

The best solution to that is to use pcntl_async_signals[true] when it's available, which will allow the signals to get caught regardless of what file the code happens to be in.

php dot net at e-z dot name ¶

8 years ago

you can register multiple tick functions:



will output on every tick:
a
b
b
b

ja2016 at wir dot pl ¶

5 years ago

Don't use uft-8 encoding with BOM. Then fatal error occurs ALWAYS. Substitute it with utf-8 without BOM.

---

*BOM*


This allows you to avoid the performance impact of a low tick value in systems which support `pcntl_async_signals[]`.

wapmorgan at gmail dot com ¶

5 years ago

If you fork'ed [with pcntl_fork] after you've used `declare`, you need to do it again in child thread.

declare[ticks=1];
$pid = pcntl_fork[];
if [$pid === 0] {
    declare[ticks=1];
} else {
    // code ..
}

aeolianmeson at NOSPAM dot blitzeclipse dot com ¶

16 years ago

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:



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.

ohcc at 163 dot com ¶

4 years ago

Using a directive that is not supported by current version of PHP will generate a warning like:

Warning: Unsupported declare 'strict_types' in D:\Server\wuxiancheng.cn\index.php on line 2

You CAN'T prepend an @ sign to the declare construct to supress that error message.

@declare[strict_types=1];

The above code will lead to a Parse error like:

Parse error: syntax error, unexpected 'declare' [T_DECLARE] in D:\Server\wuxiancheng.cn\index.php on line 2

php at niekbosch dot nl ¶

8 years ago

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:


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: aÀaß

Ionut ¶

5 years ago

I haven't found this written in the doc: when using declare in the global context, it has to be the first statement in the script, before any other output.

This works:



This doesn't work: