GravityView - Sort by Last Name when sorting by the Full Name

gravityview-sorting-full-name.php

<?php

/**
 * Override how to sort when sorting by full name.
 * 
 * Available in GravityView 1.7.4 and higher.
 * 
 * @param string $name_part `first` or `last` (default: `first`)
 * @param string $sort_field_id Field used for sorting
 * @param int $form_id GF Form ID
 * 
 * @return string Full name sorting option (`first` or `last`)
 */
function gravityview_use_last_name_sorting( $name_part, $sort_field_id, $form_id ) {
	
	// If you want to sort by different fields depending on the field ID, you can use 
	// the $sort_field_id and $form_id parameters to run some logic here, too.
	
	return 'last';
}

add_filter('gravityview/sorting/full-name', 'gravityview_use_last_name_sorting', 10, 3 );