How to set expiration date in php?

[civirol02]

<?php 
$startdate = date('Y-m-d H:i:s'); 
echo $startdate."</br>"; 
$offset = strtotime("+1 day");
$enddate = date("Y-m-d H:i:s", $offset);
echo $enddate."</br>"; 




$newamount = $availablequantity - $quantity;
if($enddate > $startdate) {


    if($availablequantity != 0) { 



$sql = mysql_query("UPDATE size SET quantity ='$newamount' WHERE id ='$item_id'") or die (mysql_error());

}
//$sql = mysql_query("UPDATE size SET quantity = WHERE size ='$item_id'") or die (mysql_error()); 

} 
else if($startdate < $enddate) {

$sql = mysql_query("UPDATE size SET quantity ='$availablequantity' WHERE id ='$item_id'") or die (mysql_error());
$sql = mysql_query("DELETE FROM reservations WHERE size_id ='$item_id'") or die (mysql_error());
}
}
}

//Can anyone help me how to return the product if no transaction has been made within a day..  What I must use in order for the system to automatically expire the product and return it to the size table. please help me. 

?>