Given this code:

class CallbackClass {
    function CallbackFunction() {
        // refers to $this
    }

    function StaticFunction() {
        // doesn't refer to $this
    }
}

function NonClassFunction() {
}

there appear to be 3 ways to set a callback function in PHP (using set_error_handler() as an example):

1: set_error_handler('NonClassFunction');

2: set_error_handler(array('CallbackClass', 'StaticFunction'));

3: $o =& new CallbackClass();
   set_error_handler(array($o, 'CallbackFunction'));

The following may also prove useful:

class CallbackClass {
    function CallbackClass() {
        set_error_handler(array(&$this, 'CallbackFunction')); // the & is important
    }
    
    function CallbackFunction() {
        // refers to $this
    }
}

The documentation is not clear in outlining these three examples.