The most basic way of connecting a signal to a callback is by using connect() method. All classes descended from GtkObject have it. For example, to respond to a 'clicked' signal on a GtkButton widget, you would do this: 

<?php
  function on_ok_clicked($button) {  }
  $ok_button->connect('clicked', 'on_ok_clicked');
?>
If you wanted to connect to a class method or an object method, you have to use a special syntax for denoting that: 

<?php
  class A {
    function on_ok_clicked($button) { }
  }
  $ok_button->connect('clicked', array('A', 'on_ok_clicked')); // connect to a class method

  $a_obj = new A();
  $ok_button->connect('clicked', array(&$a_obj, 'on_ok_clicked')); // connect to an instance method
?>