Edit SQL ORDER BY statement to randomly order posts after other orders are accounted for.

slimline-random-orderby.php

<?php

/**
 * slimline_random_orderby function
 * 
 * Edit SQL ORDER BY statement to randomly order posts after other orders are accounted for.
 * 
 * @param string $orderby The ORDER BY statement generated by get_posts()
 * @return string $orderby ORDER BY statement with RAND() set as an additional clause
 */
function slimline_random_orderby( $orderby ) {

	// Append SQL RAND function to an existing orderby statement or return by itself if $orderby is empty
	return ( $orderby ? ',' : '' ) . 'RAND()';
}

?>