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Checking

function_exists($s) returns true if the given function $s has been defined. It checks both built-in and user-defined functions. get_defined_functions() returns a multidimensional array containing all functions, both built-in and user-defined. The internal functions will be accessible via $arr[‘internal] and the user defined ones via $arr[‘user’]. 

<!DOCTYPE html><html><head></head>
<body><?php

function shout(){
   echo "Hello World";
}
var_export(function_exists('shout'));
$arr=get_defined_functions();
print_r($arr);

?></body></html>

trueArray ( [internal] => Array ( [0] => zend_version [1] => func_num_args [2] => func_get_arg ……[1392] => xmlrpc_server_register_introspection_callback ) 
[user] => Array ( [0] => shout ) )