<?php
 $x=1;
 while($x<3)
 {
     $id=strval($x);
     echo '
     <form>
     <input type="text" id="'.$id.'" value="'.$id.'">

     <input type="text" id="2_'.$id.'">
     </form>
     ';
     $x+=1;
     ?>
    <script type="text/javascript">
            pr_id(<?php echo $id; ?>,<?php echo "2_".$id; ?>);
        </script>
     <?PHP
 }
 ?>

     <script>

 function pr_id(f_id,s_id)
 {

    // some code on value of element that have id f_id

    // set result to value of element that have id s_id
 }

 </script>`