[chmod]

Thanks chmod for the answer,

How to get root on a server from the current php privilege ? do you have some reading about this ?
because i guess that doing everything you said require the root privilege isnt it, so lets say the guy got his uploadform and file-explorer php up on a machine, what are the few next step to get privilege escalation ?

This is a very broad question, yes you would need root privileges but getting this would depend on how the server is set up, what services are running (and exploitable) and the the security set up. As for reading there is plenty of choice right on this very forum try the the hacking tutorial and the website exploits categories.