php check empty or null in many variables

Hello I have a simple script

This is my script, and try this script

 <?php 
        $user_id             = $_REQUEST['user_id'];
        $pid                 = $_REQUEST['pid'];
        $nopr                = $_REQUEST['nopr'];
        $tglpr               = $_REQUEST['tglpr'];
        $uraianpr            = $_REQUEST['uraianpr'];
        $jenispr             = $_REQUEST['jenispr'];
        $nilaipr             = $_REQUEST['nilaipr'];
        $costproject         = $_REQUEST['costproject'];
        $remarkpr            = $_REQUEST['remarkpr'];
        $tmpattachid         = $_REQUEST['tmpattachid'];

if ($user_id==NULL)
{
  $error "User Id Not Complete";
}
else if ($pid==NULL)
{
  $error= "PID Not Complete";
}
echo $error;
    ?>

I want if $user_id or other variables is empty/null

i confused when other variable have empty/null data i must typing code many if statement :(

example : 5 variables ($pid,$nopr,$tglpr,$jenispr,$costproject) is empty/null

this show error like this

 PID Not Complete
 NoPR Not Complete
 Tgl Pr Not Complete
 Jenis Pr Not Complete
 Cost Project Not Complete

Help Me, Thank's.