What is a PHP callback function?
A callback (callable) function in PHP is one which is being passed as another function’s parameter—by its name as a string. Callback functions can not only be simple functions, but also object methods, including static class methods.
See the PHP manual post about this.
A method of an instantiated object is passed as an array containing an object at index
0 and the method name at index
Static class methods can also be passed without instantiating an object of that class by passing the class name instead of an object at index
0. As of PHP 5.2.3, it is also possible to pass