Accessing dynamically created variables inside a function

Hi, I'm sure this is a stupid question but I've been trying to find the solution for days and have had no luck. Hopefully someone out there can help.

So I am attempting to set a variable dynamically like so:

$pid = $GET['p_id'];

This part goes smoothly but my problem arises when I attempt to pass this variable to a function in the following way.

public function InsertUpdate($uid,$update,$uploads,$pid) {

$update=mysql_real_escape_string($update);
  $time=time();
   $ip=$_SERVER['REMOTE_ADDR'];
       $p_id = **$p_id;**

.....

        $query = mysql_query("INSERT INTO `messages` (message, uid_fk, poster_id,ip,created,uploads) VALUES ('$update', '$uid',

'$pid', '$ip','$time','$uploads')") or die(mysqlerror());

.... For whatever reason the function does not see the variable unless I set it to a constant like

$p_id = 2;

Any ideas would really be appreciated!