PHP SESSIONS deleted or modified by server - php

Question

I'm making a form for the users to fill, then I check their inputs after i 'clean'
function test_input($data)
{
$data = trim($data);
$data = stripslashes($data);
$data = htmlspecialchars($data);
return $data;
}
then, after I check some parameters I create SESSIONS to echo those values later (1 time to let the user see and check if what they input is correct, then to send an email)
Everything works fine when I do it with MAMP, but when I upload everything to the HostGator's servers some SESSIONS are being deleted and some change values! Why is this happening? And why to SOME sessions and not all of them??
Also, I use $_SESSIONS because when I validate the inputs I do it like this:
<form action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>" method="post">
<input name='id' type='text'>
</form>
So I send the input data to the same page and check it, and if everything is right I create the SESSIONS and redirect to the next form.
I know exactly where are the SESSIONS being deleted and changed their values, but I don't know how to stop it! I don't know if it's my code or something with the server.
EDIT
Here's how I save the input of one session that it's value is being modified:
I get the ID form the user from another .php:
FIRST FORM: GETTING THE ID
<?php
$checkcode = $_POST["id"];
$query = "SELECT id FROM agents_id_pass WHERE id ='$checkcode'";
$res = mysqli_query($link, $query);
$nres = mysqli_num_rows($res);
$array = mysqli_fetch_array($res);
list($id) = $array;
if ($nres == 0){
header('Location: ../compra/compra_login_fail.php');
exit();
}
else{
session_start();
$_SESSION['id'] = $checkcode; //HERE I SER THE SESSION
$_SESSION['planilla_venta_1_session'] = 1;
mysqli_close($link);
header('Location: planilla_venta_1.php');
exit();
}
SECOND FORM: GETTING PERSONAL INFO FORM THE USER (NAME, ADDRESS, ETC..)
I'm gonna show you just the session that deletes itself
<?php
session_start();
if (!isset($_SESSION['id'])){
session_destroy();
}
else{
//destroy session after 15 mins
if (isset($_SESSION['LAST_ACTIVITY']) && (time() - $_SESSION['LAST_ACTIVITY'] > 900))
{
// last request was more than 15 minutes ago
session_unset(); // unset $_SESSION variable for the run-time
session_destroy(); // destroy session data in storage
}
$_SESSION['LAST_ACTIVITY'] = time(); // update last activity time stamp
//Avoid session fixasion, changing session's id every 5 mins.
ini_set('session.gc-maxlifetime', 300);
if (!isset($_SESSION['CREATED'])) {
$_SESSION['CREATED'] = time();
} else if (time() - $_SESSION['CREATED'] > 300) {
// session started more than 5 minutes ago
session_regenerate_id(true);// change session ID for the current session an
invalidate old session ID
$_SESSION['CREATED'] = time(); // update creation time
}
}
if ($_SERVER["REQUEST_METHOD"] == "POST")
{
$zip = test_input($_POST["zip"]);
if (empty($_POST["zip"])){
$zipErr = "Código Zip es requerido";
}
else{
$zip = test_input($_POST["zip"]);
// check if name only contains letters and whitespace
if (!preg_match("/^[0-9 ]*$/",$zip))
{
$zipErr = "Sólo se permiten números";
}
else{
$_SESSION['zip'] = $zip;
}
}
if(!isset($zipErr) && isset($_SESSION['zip'])
echo "<script>
window.location = 'planilla_venta_members.php'; //IF THERE IS NO ERROR AND THE SESSION IS SET -> REDIRECT TO THE NEXT FORM. ALL SESSIONS SET
</script>";
exit();
}
}
?>
<form name="sell_form" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>" method="post">
<td style="text-align:center"><input class="input" style="width:40px; margin-right:5px" type="input" name="zip" maxlength="5" value="<?php echo $_SESSION['zip'];?>">
<br>Zip*
</td>
All my sessions are created that way, so my question is, why would the server unset or change the value of some sessions???

Answer 1

I solved it! It turns out I was writing my code with php ver 5.5 and the server php configuration was with php 5.2, so I just updated it.