Fatal error: Function name must be a string

I'm getting the following error and I don't see whats wrong with my line. Any ideas? *Fatal error*: Function name must be a string in */var/www/sites/intra-test/contract/perform.php* on line *57* and my snippet of code is: if ( $_POST["perform"] == "View Contracts" )

       {

       $mysqli_get_userid = "SELECT user_id from user where email =

'".$_SESSION["username"]."'";

       $mysqli_get_userid_result = $mysqli_query($mysqli,  // line 57

$mysqli_get_userid) or die(mysqli_error($mysqli));

       while ($userid_result = mysqli_fetch_array($mysqli_get_userid_result))
               {
               $user_id = $userid_result["user_id"];
               }

}