test.php

<?php

error_reporting(E_ALL);

class_a::init();

class class_a{
    public static $other_class_name = 'class_b';
    
    public static function init(){
        call_user_func(array(self::$other_class_name, 'init'));
    }
}
class class_b{
    public static function init(){
        echo 'i am class_b';
    }
}
class class_c{
    public static function init(){
        echo 'i am class_c';
    }
}