Grab XML line from PHP

So i'm not an PHP/XML expert but i've a simple question that i'm sure you can answer !

I'll explain :

I have a xml file which is in /requests/decoded, i need a part of it to follow my PHP script :

$accountToken =

I want him to grab these 2 parts of the XML :

<key>ActivationRandomness</key>
<string>391E262D-27A8-43AB-944F-C4F33AB12F6E</string>

<key>UniqueDeviceID</key>
<string>69348f0d170f3ef67826afa103bec669c23373cf</string>

Can you help me please ? Thanks !