mutual.php

<?php
$client = new Everyman\Neo4j\Client();

$queryString =
  "START firstPerson=node({firstId}), secondPerson=node({secondId}) " .
  "MATCH firstPerson -[:FRIEND]-> mutualFriend <-[:FRIEND]- secondPerson" .
  "RETURN mutualFriend";

$params = array(
    "firstId"  => 123,
    "secondId" => 456
);

$query = new Everyman\Neo4j\Cypher\Query($client, $queryString, $params);
$results = $query->getResultSet();