PHP：动态调用默认方法

There's the following (example) class:

class klasse
{
    private $var = 'doit';

    function doit($param)
    {
        return md5($param);
    }
    function bla($param)
    {
        // HERES THE PROBLEM
        return $this->{$this->var}($param);
    }
}

// Create new instance
$klasse = new klasse;

// Start the "dynamical output"
echo $klasse->bla('test');

This works fine! But the problem is that I'd like to call the md5() function "directly dynamically". So I don't want to go the detour with "doit()". If I try

private $var = 'md5';

at the beginning of the class I get the following (absolutely senseful) error message:

Fatal error: Call to undefined method klasse::md5() in - on line 13

So I know that this error is senseful but I have no clue how to avoid it?

How can I handle this (to directly call md5())?

Thank you!

This should work:

class klasse
{
    public function __construct() {
        $this->var = 'md5';
    }


}

$klasse = new klasse;
echo call_user_func($klasse->var, 'argument');

More info at: http://php.net/manual/en/function.call-user-func.php