Eğer id alacaksan bence name değerine id'yi ver.
yani
<input type="checkbox" name="checkbox[$urun_id]">
<input type="text" name="quanity[$urun_id]">
olsun.

<?php 
if($_POST){//to run PHP script on submit 
$qua = $_POST['quantity']; 
$cl= $_POST['check_list']; 

echo '<h2>ADDED</h2> '; 

if(!empty($cl)) 
 { 

// Loop to store and display values of individual checked checkbox. 
foreach($cl as $id=>$selected) 
 { 
 echo "ID: " .$id."&nbsp;&nbsp;\n"; 
 echo "Product : " .$selected."&nbsp;&nbsp;\n"; 
 if(isset($qua[$id])) echo "Quanity:  ".$qua[$id]."\n";
@mysql_query("update tablo set quanity = '".$qua[$id]."' where id='$id'"); 

 }   
 } 

} 
?>