Sometimes the callbacks needs access to program data or other information. To avoid messing with global variables, you can pass the additional data to the callback when you connect it. 

<?php
  function on_ok_clicked($button, $window, $my_data) { }
  $ok_button->connect('clicked', 'on_ok_clicked', $window, 'some data');
?>
connect() returns an number that uniquely identifies the connection made. This can be useful if you want to disable or remove the callback during the execution. 

<?php
  $conn_id = $ok_button->connect('clicked', 'on_ok_clicked');
  $ok_button->signal_handler_block($conn_id); // disable callback
  $ok_button->signal_handler_unblock($conn_id); // re-enable callback
  $ok_button->disconnect($conn_id); // remove callback permanently
?>