iOS "corrupting" PHP Session - php

Question

I have been working on a bug now that has been plaguing me for ages and it is really obscure and difficult to get to the bottom of.
The bug only affects iOS devices (iPhone/iPad)
It won't always affect a device, it seems to be random
Customers who have placed several orders fine may suddenly experience it, then when placing another order minutes later not experience it
We have seen it happen in both very old devices and newer versions of iOS
It only affects users using Safari that we have seen
Theories we have tested:
Something to do with mobile data/switching between cell towers when roaming causes the session to become corrupt. We were unable to reproduce this and customers experiencing it say it has happened while at home on WiFi
When switching between HTTP/HTTPS the session is becoming void, we made the entire site secure and it still happened
Poor code in our cart somewhere, we operate multiple brands and some are using different carts. A similar thing happens in all of them
Server misconfiguration, tried multiple servers with 4 different hosts. They configured the server in 2 instances with the same result.
The payment gateway isn't working correctly (we used to use a HPP & PayPal solution), we switched to an API solution with the same result
Somehow misconfigured product, can't be the case as the same product works fine in almost all cases, except the odd iOS ones
Below I have included a sample of what the array should look like storing the cart, and what it looks like on a corrupt iOS cart
Correct Cart
$_SESSION['cart'] = array(
'items' = array (0 => array(
'product_name' => 'Shirt',
'product_price' => '20.00',
'product_sku' => '801245',
'vat_rate' => '20.0000',
'attributes' => array(
'size' => array('value' => 'large', 'cost' => '+5'),
'qty' => array('value' => 1, 'cost' => '+0'),
'colour' => array('value' => 'blue', 'cost' => '+0'),
),
'delivery_info' => array(
'name' => 'Mr A. Tester',
'line1' => 'Street',
'line2' => 'Off another street',
'company' => 'ACME Ent.',
'postcode' => 'AA1 0AA',
'county' => 'London',
'country' => 'United Kingdom',
'date' => '2015-12-01',
),
)
),
'totals' = array (
'item' => '20.00',
'extras' => '5.00',
'delivery' => '3.00',
'blended_vat' => '20.0000',
'to_pay' => '28.00',
),
'cart_id' = 'fc5e038d38a57032085441e7fe7010b0',
'customer_id' = '102145',
'order_status' = 'Pending',
'paid' = false,
)
Corrupt iOS Cart
$_SESSION['cart'] = array(
'items' = array (
'attributes' => array(
'size' => array('value' => 'large', 'cost' => '+5'),
'qty' => array('value' => 1, 'cost' => '+0'),
'colour' => array('value' => 'blue', 'cost' => '+0'),
),
'delivery_info' => array(
'name' => 'Mr A. Tester',
'line1' => 'Street',
'line2' => 'Off another street',
'company' => 'ACME Ent.',
'postcode' => 'AA1 0AA',
'county' => 'London',
'country' => 'United Kingdom',
'date' => '2015-12-01',
),
)
),
0 => array(
'product_name' => 'Shirt',
'product_price' => '20.00',
'product_sku' => '801245',
'vat_rate' => '20.0000',
'totals' = array (
'item' => '20.00',
'extras' => '5.00',
'delivery' => '3.00',
'blended_vat' => '20.0000',
'to_pay' => '28.00',
),
'cart_id' = 'fc5e038d38a57032085441e7fe7010b0',
'customer_id' = '102145',
'order_status' = 'Pending',
'paid' = false,
)
I would love to hear anyone's thoughts on this and any possible suggestions. As I say the cart works fine with every OS/Browser combination. It only randomly seems to affect iPhones/iPads, and could work fine on the same device several times, then not work, then work again.
It seems random as to when it won't work.