Question

I am 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
    }
}

Answer 1

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.