you pass it in the URL like this:
in page1.php
$result = mysql_query("SELECT dep_no FROM employees where emp_id = $id",$db);
$url = "page2.php?dep_no=" . mysql_result($result,'dep_no');
header("Location: $url");
in page2.php
<input type="hidden" name="dep_no" value="<?php echo $_GET['dep_no']; ?>">