Anyone could help me with a code to detect a user mobile device. Just like fb where it says ''download fb chat for your Nokia bla bla'' and its unique for ever user. H0pe u get pls

echo $_SERVER["HTTP_USER_AGENT"];
you need to explode and get the first part.
Vary very simple(sorry to say that) have you ever tried studying the output of the phpinfo(); function call. It contains virtually all the information sent in the header. Just create a new php file with the single line of code.
phpinfo();
examine the output there are many global variables containing your computer info in the list just find the one with your computer's name. If you echo that variable anywhere on any page any how it will supply the specific device name. Same for browser name and version and many more

^Exactly.

@OP, also don't forget to make provision for unavailable user-agents, that is user agents that are not recognized should have a defined/default behaviour/output.