is_callable() invoked on system functions

[kwumr]

I have a plugin named Ord. When using 'plugin' => 'Ord' in my permissions array, the _matchPermission() method evaluates is_callable('Ord') as true and attempts to call_user_func().

ord() is a built-in PHP function so is_callable() returns true. Additional checks on $value should be performed: if(!is_string($value) and is_callable($value))

[steinkel]

closed per #48