function_exists

(PHP 4, PHP 5, PHP 7, PHP 8)

function_exists — 如果给定的函数已经被定义就返回 true

说明

function_exists(string $function): bool

在已经定义的函数列表（包括系统自带的函数和用户自定义的函数）中查找 function。

参数

function: 函数名，必须为一个字符串。

返回值

如果 function 存在且的确是一个函数就返回 true，反之则返回 false。

注意:
对于语法结构的判断，例如 include_once 和 echo 将会返回 false。

示例

示例 #1 function_exists() 的例子

<?php
if (function_exists('imap_open')) {
    echo "IMAP functions are available.<br />\n";
} else {
    echo "IMAP functions are not available.<br />\n";
}
?>

注释

注意:
即使函数本身由于配置或者编译选项而无法使用，该函数名也可能存在（image 就是一个现成的例子）。

参见

method_exists() - 检查类的方法是否存在
is_callable() - 验证值是否可以在当前范围内作为函数调用。
get_defined_functions() - 返回所有已定义函数的数组
class_exists() - 查类是否已经定义
extension_loaded() - 检查一个扩展是否已经加载

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＋添加备注

用户贡献的备注 19 notes

down

kitchin ¶

12 years ago

You can use this function to conditionally define functions, see: http://php.net/manual/en/functions.user-defined.php



For instance Wordpress uses it to make functions "pluggable." If a plugin has already defined a pluggable function, then the WP code knows not to try to redefine it.



But function_exists() will always return true unless you wrap any later function definition in a conditional clause, like if(){...}. This is a subtle matter in PHP parsing. Some examples:



<?php

if (function_exists('foo')) {

  print "foo defined\\n";

} else {

  print "foo not defined\\n";

}

function foo() {}



if (function_exists('bar')) {

  print "bar defined\\n";

} else {

  print "defining bar\\n";

  function bar() {}

}

print "calling bar\\n";

bar(); // ok to call function conditionally defined earlier



print "calling baz\\n";

baz(); // ok to call function unconditionally defined later

function baz() {}



qux(); // NOT ok to call function conditionally defined later

if (!function_exists('qux')) {

  function qux() {}

}

?>

Prints:

  foo defined

  defining bar

  calling bar

  calling baz

  PHP Fatal error: Call to undefined function qux()



Any oddities are probably due to the order in which you include/require files.

down

michael at squiloople dot com ¶

9 years ago

It should be noted that the function_exists check is not relative to the root namespace. This means that the namespace should be appended to the check:

<?php

  namespace test;

  if (!function_exists(__NAMESPACE__ . '\example'))
  {

    function example()
    {

    }

  }

?>

down

chris at candm dot org dot uk ¶

3 years ago

The confusion expressed in some of the submissions here arise because functions declared outside conditional blocks are defined as the code is loaded and are thus callable and exist wherever in the code they are declared, whereas those declared inside a condition block are not defined until that block is executed. Thus:

    echo foo();
    function foo() { return "I am foo"; }

yields : "I am foo"

however because those inside a conditional are defined as they are encountered during code execution

    echo foo();
    if(true) {
        function foo() { return "I am foo"; }
    }

yields: Fatal error: Uncaught Error: Call to undefined function foo()

down

ayyappan dot ashok at gmail dot com ¶

8 years ago

PHP supports nested function based on certain criteria.

Please look over the code.

function Media(){ }

function Audio()
{
    echo "Plugged Audo 5.1:<br/>";
    function Volume()
    {
    echo "Volume controls:<br/>";
    function Equalizer()
    {
        echo "Equalize Bands:<br/>";
    }
   }
}
//Call to nested functions
Audio();
Volume();
Equalizer();

if(function_exists('Volume')):
echo "TRUE";
else:
echo "FALSE";
endif;

Case 1:  //Result :Works Well
--------
Audio();
Volume();
Equalizer();

Case 2:  //Results Notice Error. Root function Audio must be called first.
--------
Volume();

Case 3:  //Results Error. Root function Volume must be called.
--------
Audio();
Equalizer();

Note : 
The nested function should be called based on their order used.
In our example when Audio is not called and instantly when we try to call Volume puts under error. 

Even though there is an possibility to use nested functions in PHP. It looks overhead to do so. Better to avoid in logical ground of script.

Tested on PHP 5.5.32

down

andi at splitbrain dot org ¶

18 years ago

function_exists will return false for functions disabled with the disable_functions ini directive. However those functions are still declared so trying to define them yourself will fail.



<?php

if(!function_exists('readfile')){

  function readfile($file){

    $handle=@fopen($cache,"r");

    echo @fread($handle,filesize($file));

    @fclose($handle);

  }

}

?>



The above will issue a "Cannot redeclare readfile()" fatal error if readfile was disabled with disable_functions.

down

fili at fili dot nl ¶

19 years ago

To prevent direct calls to included files i use the following technique.

In the main file create an empty function with a random name. Like so:

<?php
function hjudejdjiwe() { return true; }
?>

Then check for the existence of this function within your include:

<?php
if (!function_exists('hjudejdjiwe')) { die('!'); }
?>

Simple but effective.

down

BruceB ¶

9 years ago

This is not going to go down as you might expect it should (even if you play smart and require/include_once):

<?php
if(function_exists('my_function'))
{
   throw new Exception("'my_function' is already defined!");
}

function my_function()
{
   // Do the work here
}
?>

This, however does work:

<?php
if( ! function_exists('my_function'))
{
   function my_function()
   {
      // Do the work here
   }
}
else
{
   throw new Exception("'my_function' is already defined!");
}
?>

Does it have anything to do with PHP parse/execute phases or global/local scope or those curly brackets or something else, I have no idea, but the latter ugly son works, while the former bombs out claiming that 'my_function' is already defined.

Thought this might save someone a few minutes of debugging time...

down

admin at gk-root dot com ¶

12 years ago

// If you want to chack if the function is enabled or disable in php.ini you can use this function:

<?php
function func_enabled($func) {
    $disabled = explode(',', ini_get('disable_functions'));
    foreach ($disabled as $disableFunction) {
        $is_disabled[] = trim($disableFunction);
    }
    if (in_array($func,$is_disabled)) {
        $it_is_disabled["m"] = $func.'() has been disabled for security reasons in php.ini';
        $it_is_disabled["s"] = 0;
    } else {
        $it_is_disabled["m"] = $func.'() is allow to use';
        $it_is_disabled["s"] = 1;
    }
    return $it_is_disabled;
}
?>

// An example of how to use:

<?php
$if_is_disabled = func_enabled('exec'); // return Arrey
echo $if_is_disabled["m"]; // return text value
echo '<br/>'; 
echo $if_is_disabled["s"]; // return 1 or 0
?>

down

webmaster at mamo-net dot de ¶

17 years ago

If you use suhosin.executor.func.blacklist instead of disabled_functions in your php.ini, function_exists will return true for a disabled function. I used this to have the same beahviour with suhosin.executor.func.blacklist and disabled_functions:

<?php
function suhosin_function_exists($func) {
    if (extension_loaded('suhosin')) {
        $suhosin = @ini_get("suhosin.executor.func.blacklist");
        if (empty($suhosin) == false) {
            $suhosin = explode(',', $suhosin);
            $suhosin = array_map('trim', $suhosin);
            $suhosin = array_map('strtolower', $suhosin);
            return (function_exists($func) == true && array_search($func, $suhosin) === false);
        }
    }
    return function_exists($func);
}
?>

down

ckrack at i-z dot de ¶

20 years ago

i was wondering whether is_callable or function exists is faster when checking class methods.

is_callable(array('foo', 'bar'));
function_exists('foo::bar');

my results when doing each operation 10000 times with a simple test class were the following:

is_callable: 0.28671383857727 seconds
function_exists: 0.14569997787476 seconds

(following tests have proved this to be true).

thus you can see, function_exists is twice as fast as is_callable.

down

-1

breadman ¶

21 years ago

Functions within a function are better off as anonymous returns from create_function(), unless you want to be able to call it elsewhere.

However, I have used this in skinning:  I use alert_box() to display certain errors, like a faulty SQL query.  This simply calls display_alert(), which is defined in my skin scripts.  However, alert_box() is sometimes called before I know which skin to load, so it has its own functionality which it uses if function_exists('display_alert') returns false.

down

-1

Dan ¶

18 years ago

I would like to comment on the following post:

A note of caution: function_exists() appears to be case-insensitive (at least as of PHP 4.3.8).  e.g.:

<?php
   function MyCasedFunction() {
       return true;
   }

   // Will return true, even though casing is "wrong"
   if (function_exists("mYcAsEdFuNcTiOn"))
       echo "I see it!";
?> 

I believe that function calls itself are case insensitve, so this function is returning a valid truth. PHP doesn't care about cases.

down

-2

neelam_ab2003 at yahoo dot co dot in ¶

18 years ago

<?php
/*PHP doesn't Support nested functions. I have tried following in PHP_VERSION - 5.1.2*/

function A(){}

function B(){
    function C(){
        function D(){}
    }
}

IsFunctionExist('A');
IsFunctionExist('B');
IsFunctionExist('C');
IsFunctionExist('D');

function IsFunctionExist($funcName){
    echo function_exists($funcName)?" $funcName exist <br>":" $funcName doesn't exist <br>";
}
?>

/*O U T P U T
A exist 
B exist 
C doesn't exist 
D doesn't exist 
*/

down

-3

bob at thethirdshift dot net ¶

20 years ago

I, too, was wondering whether is_callable or function exists is faster when checking class methods.  So, I setup the following test:

<?php
function doTimes($start, $end)
  {
    $start_time = explode (" ", $start);
    $start_time = $start_time[1] + $start_time[0];
    $end_time = explode (" ", $end);
    $end_time = $end_time[1] + $end_time[0];
    $time = $end_time - $start_time;
    return $time;
  }

class test
  {
      function test()
      {
          return true;
      }
  }
  
$callableIsTrue = false;
$startIsCallable = microtime();
for($i = 0; $i < 10000; $i++)
  {
      if(is_callable(array('test', 'test'))) { $callableIsTrue = true; }
  }
$endIsCallable = microtime();

$existsIsTrue = false;
$startExists = microtime();
for($i = 0; $i < 10000; $i++)
  {
      if(function_exists('test::test')) { $existsIsTrue = true; }
  }
$endExists = microtime();

$timeIsCallable = doTimes($startIsCallable, $endIsCallable);
$timeExists     = doTimes($startExists, $endExists);

echo "<b>is_callable = ".($callableIsTrue ? "TRUE" : "FALSE")."</b>, \n";
echo "<b>function_exists = ".($existsIsTrue ? "TRUE" : "FALSE")."</b><br>\n";

echo "<br>Did 10000 is_callables in ".$timeIsCallable." seconds";
echo "<br>Did 10000 function_exists in ".$timeExists." seconds";
?>

This gives the output :

is_callable = TRUE, function_exists = FALSE

Did 10000 is_callables in 0.0640790462494 seconds
Did 10000 function_exists in 0.0304429531097 seconds

So the fact that function_exists is twice as fast is slightly over shadowed by the fact that it doesn't work on class methods, at least not as far as I can tell.

down

-2

dshearin at excite dot com ¶

21 years ago

This can be used to conditionally define a user function. In this sense, it can act as a sort of inline include_once(). 



For example, suppose you have a function A that calls function B. B is only used inside function A and is never called from anywhere else in the script. It's logical (and perfectly legal in PHP) to define B inside of A's definition, like so: 



<?php

function A($inputArray)

{

   if (!function_exists('B'))

   {

      function B($item)

      {

           // Do something with $item

         // and return result

          return $result;

      }

   }

   foreach ($inputArray as $nextItem) $outputArray[] = B($nextItem);

   return $outputArray;    

}

?>



Without the function_exists test, you would get a fatal error the second time you called A, as PHP would think you were trying to redefine B (not legal in PHP). The placement of the test is also important. Since the if block is executed sequentially, like any other block of code, it must come before any call to the function defined within.

down

-4

brooklynphil at hotmail dot com ¶

17 years ago

to bob at thethirdshift dot net
regarding is_callable vs function_exists.

using your code
is_callable = TRUE, function_exists = FALSE

Did 10000 is_callables in 0.0443360805511 seconds
Did 10000 function_exists in 0.0111110210419 seconds

then we replace
is_callable(array('test','test'));
with
$callarray = array('test','test'); // place this outside for-loop
is_callable($callarray);

is_callable = TRUE, function_exists = FALSE

Did 10000 is_callables in 0.0314660072327 seconds
Did 10000 function_exists in 0.0120670795441 seconds

then we replace
is_callable(array('test','test'));
with
is_callable('test','test');

is_callable = TRUE, function_exists = FALSE

Did 10000 is_callables in 0.00991606712341 seconds
Did 10000 function_exists in 0.0113790035248 seconds

I hope you can see that loop-testing functions is not so simple. :)

down

-5

php at fluidthoughts dot com ¶

16 years ago

function_exists returns false on NULL and empty string:

<?php
        if (function_exists('')) {
                echo "empty string function exists\n";
        }

        if (function_exists(NULL)) {
                echo "NULL function exists\n";
        }
?>

Neither of the echo statements happen when I run this.

down

-5

White-Gandalf ¶

17 years ago

I stumbled over the same problem as "eddiec" (users not able or not willing to use "_once"-suffixes).

A possible alternative explanation for the behavior:

If a file is included, it is possibly parsed every include-time.(?)
While parsing, every function in global scope is tried to register. THIS gets wrong, when multiple times included, and it produces an error.

If functions are defined within block scopes, their registration seems to be delayed until execution of such a block. Thus, not the function "function_exists" functions wrong, but simply the philosophy of the interpreter produces such results.

Thus, the same effect can be achieved by simply putting block braces around the contents of an include_once file:

if (function_exists('function_in_question')) return;
{
    function function_in_question(...)
    {
        ...
    }
    ...other stuff
}

...which is equivalent to...

if (!function_exists('function_in_question'))
{
    function function_in_question(...)
    {
        ...
    }
    ...other stuff
}

down

-4

b dot g dot dariush at gmail dot com ¶

10 years ago

function_exists() does not cache its query internally.

by executing the following code

        $funcs = \array_shift(\get_defined_functions());
        $l = new \core\utiles\loadTime;
        $times = 0;
        $l->start();
        for($index = 0; $index<count($funcs); $index++)
        {
            foreach($funcs as $func)
            {
                $times++;
                if(\function_exists($func)) ;
            }
        }
        $s = $l->stop();
        echo "<span style='color:green'>$times</span> took : $s";
        # the output would be 
         $> 2365444 took : 0.70324

By executing the 

        $funcs = \array_shift(\get_defined_functions());
        $l = new \core\utiles\loadTime;
        $times = 0;
        $l->start();
        static $func_check = array();
        for($index = 0; $index<count($funcs); $index++)
        {
            foreach($funcs as $func)
            {
                $times++;
                if(!isset($func_check[$func]))
                {
                    if(\function_exists($func)) ;
                    $func_check[$func] = 1;
                }
                else $func_check[$func];
            }
        }
        $s = $l->stop();
        echo "<span style='color:green'>$times</span> took : $s";
        # the output would be 
        $> 2365444 took : 0.53446

There is a 0.16878 seconds improvement, when you use static array to cache methods existence.