Código PHP:
<?php
session_start();
$link=mysql_connect("localhost","root","");
mysql_select_db("web",$link);
$id_form = $HTTP_GET_VARS["id"];
$query = 'SELECT * FROM registro WHERE $id_form = $_GET['id'] limit 1?';
$fila = mysql_query($query,$link);
$modificacion= mysql_fetch_assoc($fila);
?>