<?php
$items = array('a', 'b', 'z');

function render_array_as_serial_comma($items) {
	$final_string = array_pop($items);
	if(count($items) >= 1) {
		$final_string = implode(', ', $items) . ' and ' . $final_string;
	}
	return $final_string;
}

echo render_array_as_serial_comma($items);