Callback.php
public function isCallable()
    {
        if ($this->isCallable === null) {
            $this->isCallable = is_callable($this->native, false);
        }
        return $this->isCallable;
    }
public function isCallable()
    {
        if ($this->isCallable === null) {
            $this->isCallable = is_callable($this->native, false);
        }
        return $this->isCallable;
    }
public function isCallable ( ) { if ( $ this -> isCallable === null ) { $ this -> isCallable = is_callable ( $ this -> native , false ) ; } return $ this -> isCallable ; }
Checks if the callback is callable

@return boolean
Checks if the callback is callable