PHP variables do not exist between pages you will either have to use session variables, cookies or URL vairiable to pass the information back.
The easiest might be to put the values into the URL. You would have to build your link for the return as follows:
$href = "AvailableRooms.php?dayArrive=" . $dayArrive . "&month=" . $month . "&year=". $year . "&nights=" . $nights;
Then change you link to be more like this
<a href="<?php echo $href ?>" target="_self" >Return to previous page</a>
This will create a link that looks like this.
<a href="AvailableRooms.php?daysArrive=15&month=5&year=2011&nights=6" target="_self" >Return to previous page</a>
Now the information is being passed back to the previous page correctly. You will have to also retrieve this information from the corresponding $_GET array just like you did with the post
$dayArrive = $_GET["dayArrive"];
$month = $_GET["month"];
$year = $_GET["year"];
$nights = $_GET["nights"];
You will also need to use
isset($_GET["dayArrive"]) to test to see if you are getting data back or is this a first time display of the page.
You should look at the urlencode PHP function to make sure you do not have any issues passing information this way.