<form action="process.php" method="post">
<select name="color">
<option value="red">Red</option>
<option value="blue">Blue</option>
</select>
<input type="submit" value="Submit">
</form>

$color = $_POST['color'];

if(isset($_POST['color'])) {
$color = $_POST['color'];
}

$query_string = "color=" . urlencode($color);
header("Location: process.php?" . $query_string);