call_user_func() expects parameter 1 to be a valid callback

Question

I'm just playing around with the call_user_func function in PHP and am getting this error when running this simple code:

<?php


class A
{

    public $var;
    private function printHi()
    {

        echo "Hello";   

    }

    public function __construct($string)
    {
        $this->var = $string;   


    }

    public function foo()
    {

        call_user_func($this->var); 

    }

}

$a = new A('printHi');
$a->foo();


?>

I know that if I make a function outside the class called printHi, it works fine, but I'm referring to the class's print hi and not sure why the "this" isn't being registered.

Answer 1

$this->var is evaluating to printHi in your example. However, when you are calling a method of a class, you need to pass the callback as an array where the first element is the object instance and the second element is the function name:

call_user_func(array($this, $this->var));

Here is the documentation on valid callbacks: http://www.php.net/manual/en/language.types.callable.php

Answer 2

Alternatively to Omar's answer, you can also make printHi() a class static function, so you then can call it from call_user_func('A::printHi') , like this:

class A
{

    public $var;
    public static function printHi()
    {

        echo "Hello";   

    }

    public function __construct($string)
    {
        $this->var = $string;   


    }

    public function foo()
    {

        call_user_func($this->var); 

    }

}

$a = new A('A::printHi');
$a->foo();

See live example