What exactly is the difference between the is_callable and function_exists in PHP?

I am 开发者_C百科working on a project, in which I am using a deprecated function from the older version. But don't want my script to stop if used in the older version.

So I am checking if the function exists and if not then create it.

What is the difference between function_exists and is_callable in PHP and which one is better to use?

if (!is_callable('xyz')) {
    function xyz() {
       // code goes here
    }
}

OR

if(!function_exists('xyz')) {
    function xyz() {
      // code goes here
    }
}

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The function is_callable accepts not only function names, but also other types of callbacks:

• Foo::method
• array("Foo", "method")
• array($obj, "method")
• Closures and other invokable objects (PHP 5.3)

So is_callable accepts anything that you could pass call_user_func and family, while function_exists only tells if a certain function exists (not methods, see method_exists for that, nor closures).

Put another way, is_callable is a wrapper for zend_is_callable, which handles variables with the pseudo-type callback, while function_exists only does a hash table lookup in the functions' table.

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When used with a function (not a class method) there is no difference except that function_exists is slightly faster.

But when used to check the existence of methods in a class you cannot use function_exists. You'll have to use is_callable or method_exists.

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When used in class context, is_callable returns true for class methods that are accessible ie public methods but method_exists returns true for all methods - public, protected and private. function_exists does same thing as method_exists outside class contexts.

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If a function Plop exists then function_exists("Plop") will return true.

See function_exists

If a variable is callable then is_callable($var) will return true.
Now this could mean that $var is a function name.
But i could also be an object and method name combo.

See is_callable

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One more different:

If you have a class which uses __call magic method, the is_callable($object, $method) will always return true for anything because __call basically accepts every method name. And as you might already know, method_exists will return false for this if the method is not defined in the class.

This is especially annoying if you use CakePHP Model class as they implemented __call magic method to provide magic queries, but not cool if you want the method to be absolutely defined.