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action is not described

in_widget_form action-hook . WP 2.8.0

Fires at the end of the widget control form.

Use this hook to add extra fields to the widget form. The hook is only fired if the value passed to the 'widget_form_callback' hook is not false.

Note: If the widget has no form, the text echoed from the default form method can be hidden using CSS.

Usage

add_action( 'in_widget_form', 'action_function_name_7331', 10, 3 );
function action_function_name_7331( $this, $return, $instance ){
	// action...
}
$this(WP_Widget)
The widget instance (passed by reference).
$return(null)
Return null if new fields are added.
$instance(array)
An array of the widget's settings.

Where the hook is called

WP_Widget::form_callback()
in_widget_form
wp-includes/class-wp-widget.php 525
do_action_ref_array( 'in_widget_form', array( &$this, &$return, $instance ) );

Where the hook is used (in WP core)

Does not used.