Remember that containers are widgets that can contain other widgets. The simplest type of container is GtkBin - it can only hold one child widget. An example of this container is a regular button.

To add to or remove a widget from such a container:

<?php
  $container->add($widget);
  $container->remove($widget);
?>

To obtain all the widgets in a container, use one of the following approaches.

<?php
  $child_widgets = $container->children();
  $child_widgets = $container->children;
?>

In either case $child_widgets will be an array of immediate children, but the second approach lets you access a specific child directly, e.g. $container->children[0].