<?php
$xml = new SimpleXMLElement(file_get_contents('sushi.xml'));

$fishname = 'Squid';
$variationname = 'With Wasabi';

$fish = $xml->xpath("/sushi/fish[@name='$fishname']");

$fish2 = $fish[0];

$variation = $fish2->xpath("variations/variation[@name='$variationname']");

$variation2 = $variation[0];

echo (string)$variation2->price;
?>