php man page for function_exists

FUNCTION_EXISTS(3)							 1							FUNCTION_EXISTS(3)

function_exists - Return TRUEif the given function has been defined

SYNOPSIS
       bool function_exists (string  $function_name)

DESCRIPTION
	Checks the list of defined functions, both built-in (internal) and user-defined, for $function_name.

PARAMETERS
	      o $function_name
		- The function name, as a string.

RETURN VALUES
	Returns TRUE if $function_name exists and is a function, FALSE otherwise.

       Note

	       This function will return FALSE for constructs, such as include_once(3) and echo(3).

EXAMPLES
       Example #1

	      function_exists(3) example

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

NOTES
       Note

	       A  function  name may exist even if the function itself is unusable due to configuration or compiling options (with the image func-
	      tions being an example).

SEE ALSO
       method_exists(3), is_callable(3), get_defined_functions(3), class_exists(3), extension_loaded(3).

PHP Documentation Group 													FUNCTION_EXISTS(3)