[PHP] Changeing user

[Davíð Örn Jóhannsson]

I need to be able to create dirs, chmod and other stuff on the server,
but I get : Warning: MkDir failed (Permission denied), so 
Is there any way for me to do something so I can change the user I
attept to execute those functions.
 
Something like exec(change user, pass).
 
Thanks David