Session data not being saved on some mobile versions of Safari - authentication

Question

It works just fine on my iPhone 3GS, but several employees with iPhone 4 and up are not able to log into my site through safari because their login credentials are not being saved to $_SESSION. If they download chrome onto their phones it works.
Any idea how to get this working?
When a user attempts to log in, their credentials are first verified as being accurate, and when they are they are sent to:
validate_login.php
<?php
session_start();
// sent from the login form
// employee id and username
$employeeId = $_POST['employee_id'];
$password = $_POST['password'];
//make connection to database, bail if no connection
$connection = odbc_pconnect('workorders','','');
if (!$connection) { exit("Connection Failed: " . $connection); }
//retrieve usernames and passwords
$sql = "SELECT * FROM LoginTable WHERE EmployeeID='$employeeId' and Password='$password'";
$rs = odbc_exec($connection, $sql);
if (!$rs) { exit("Error in SQL"); }
if(odbc_fetch_row($rs)) {
// register employeeId and password in session cookie and redirect to homepage
$_SESSION['employee_id'] = $employeeId;
header("location:../../home.php");
}
else {
header("location:../../login.php?error=0");
}
//close connection
odbc_close($connection);
?>
This is about as simple as you can be... any idea why it wouldn't be working? When I do a check at the top of every page that they are logged in:
<?php
session_start();
//validate user logged in; if not, redirect to login
if(!isset($_SESSION['employee_id'])){
header("location:login.php");
}
?>
It just thinks they're not logged in and throws them back out to login.php
Again, this is just an issue on Safari for newer iPhones as far as I can tell. Suggestions?