I think you forgot the echo :)
<?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);