php - Session under class -

hello stuck in syntax, can me next code?

class user {       protected $userid = preg_replace('#[^0-9]#i', '', $_session['user_id']);   protected $useremail = preg_replace('#[^a-za-z0-9@_.-]#i', '', $_session['user']);   protected $userpassword = preg_replace('#[^a-za-z0-9]#i', '', $_session['user_password']);    public function checkuserlogin(){          if(!isset($_session['user'])){       header("location: login.php");       exit();         }      //try find user session data in database     $sql = "select * users id = '$this->userid' , email = '$this->useremail' , password = '$this->userpassword' limit 1";     $res = mysql_query($sql) or die(mysql_error());     $usermatch = mysql_numrows($res);     if ($usermatch == 0) {       header("location: login.php");       exit();     }   } } 

first of all, note when declaring property, cannot assign value that's not know @ compile-time -- means cannot call function initialize property.

this code :

protected $userid = preg_replace('#[^0-9]#i', '', $_session['user_id']); 

is not valid.

reference, can read properties page of manual (quoting relevant sentence) :

they defined using 1 of keywords public, protected, or private, followed normal variable declaration.

this declaration may include initialization, this initialization must constant value -- is, must able evaluated @ compile time , must not depend on run-time information in order evaluated.

you should first declare property ; and, later (in constructor of class, instance), initialize :

class user {      protected $userid;      public function __construc() {         $this->userid = preg_replace('#[^0-9]#i', '', $_session['user_id']);     } }