Clean up `$prefix` generation in install.php
Clean up `$prefix` generation in install.php

SoftCreatR/WCF,Cyperghost/WCF,WoltLab/WCF,SoftCreatR/WCF,Cyperghost/WCF,WoltLab/WCF,SoftCreatR/WCF,Cyperghost/WCF,WoltLab/WCF,Cyperghost/WCF,SoftCreatR/WCF,Cyperghost/WCF,WoltLab/WCF