<?php
$userPath = "../../../../";

$storagePath = "/var/www/mysite/uploads";
$path = $storagePath . $userPath;

$path = realpath($path);
if(strpos($path, $storagePath) === 0){
   //Path is okay
   echo "Okay";
}
else {
   //User wants to gain access into a forbidden area.
   echo "Danger";
}

Danger