Useful for creating a comma separated list from array with "&" before the last element.

functions.php

<?php
/**
 * Join a string with a natural language conjunction at the end.
 *
 * Useful for creating a comma separated list from array with "&" before the
 * last element.
 *
 * @author Daniel James
 * @link   https://goo.gl/kQK5ke
 *
 * @param array  $list
 * @param string $conjunction Optional. Alternative values could be 'and' and 'or'. Default is an ampersand.
 *
 * @return mixed|string
 */
function iweb_natural_language_join( array $list, $conjunction = '&amp;' ) {
	$last = array_pop( $list );

	if ( $list ) {
		return implode( ', ', $list ) . ' ' . $conjunction . ' ' . $last;
	}

	return $last;
}