Tracking OpenCart checkout events accurately is critical for ad attribution, conversion optimization, and remarketing. By routing checkout data server-side using Server-Side Google Tag Manager (ssGTM), you gain resilient, ad-block-proof, and privacy-compliant tracking for platforms like GA4, Google Ads, Meta, and others.

✅ Why Send Checkout Events Server-Side?

Benefit	Description
Ad Blocker Resilience	Bypasses client-side tag blockers
Accurate Attribution	Ensures gclid/fbp match and timestamp
Privacy Control	Filters/cleans data before reaching vendors
Enhanced Matching	Passes hashed emails and first-party identifiers
Conversion Deduplication	Server-side logic avoids double firing

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🧰 Requirements

• OpenCart store (v3.x or v4.x)
• Server-Side GTM container deployed (e.g. via Stape or GCP)
• Web GTM container already installed
• GA4 and Google Ads accounts with conversion actions configured

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🚀 Step-by-Step Implementation

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🔹 Step 1: Capture Checkout Event in OpenCart (PHP)

Edit: catalog/controller/checkout/success.php

After successful order creation, extract transaction data:

$order_id = $this->session->data['order_id'];
$order_info = $this->model_checkout_order->getOrder($order_id);
$products = $this->model_checkout_order->getOrderProducts($order_id);

$items = [];
foreach ($products as $product) {
    $items[] = [
        'item_id' => $product['model'],
        'item_name' => $product['name'],
        'price' => $product['price'],
        'quantity' => $product['quantity']
    ];
}

$checkout_event = [
    'event' => 'purchase',
    'ecommerce' => [
        'transaction_id' => $order_info['order_id'],
        'value' => $order_info['total'],
        'currency' => $order_info['currency_code'],
        'items' => $items
    ],
    'user_data' => [
        'email' => $order_info['email'],
        'phone' => $order_info['telephone']
    ]
];

Then pass to Twig for frontend use:

$data['gtm_purchase'] = json_encode($checkout_event);

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🔹 Step 2: Push Checkout Event to Data Layer (Client Side)

In success.twig:

{% if gtm_purchase %}
<script>
window.dataLayer = window.dataLayer || [];
dataLayer.push({{ gtm_purchase|raw }});
</script>
{% endif %}

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🔹 Step 3: Send Checkout Event to Server GTM (via Web GTM)

In Web GTM, create:

GA4 Purchase Event Tag:

• Event Name: purchase
• Event Parameters:
  • transaction_id, value, currency, items
• Under Fields to Set, add:

Field Name	Value
transport_url	https://gtm.yoursite.com

✅ This sends event to your Server GTM endpoint.

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🔹 Step 4: Server GTM – Create GA4 Tag

In Server GTM Container:

1. Tag Type: Google Analytics: GA4
2. Trigger: Event Name equals purchase
3. Parameters: Map from Event Data:
   • transaction_id, value, currency, items

This delivers the event to GA4 via measurement protocol.

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🔹 Step 5: Server GTM – Create Google Ads Conversion Tag

In Server GTM:

1. Tag Type: Google Ads Conversion
2. Conversion ID/Label: From Google Ads UI
3. Event Parameters:
   • transaction_id: {{Event Data → ecommerce.transaction_id}}
   • value: {{Event Data → ecommerce.value}}
   • currency: {{Event Data → ecommerce.currency}}
4. Trigger: When Event Name = purchase

✅ This ensures conversion is sent server-side, even if user blocks JS.

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🔹 Step 6: Optional – Enhance with User Matching

In user_data, include hashed values (email, phone) to improve match rates:

PHP Hashing:

$email_hashed = hash('sha256', strtolower(trim($order_info['email'])));
$phone_hashed = hash('sha256', preg_replace('/\D/', '', $order_info['telephone']));

Pass this in the user_data block:

'user_data' => [
    'email' => $email_hashed,
    'phone' => $phone_hashed
]

Then access in Server GTM via Event Data variables for Meta CAPI or Google Ads Enhanced Conversions.

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🔹 Step 7: Deduplicate Conversions with event_id

Add a unique ID in PHP:

'event_id' => uniqid('oc_', true)

Send this to both:

• Client-side GA4 tag (in GTM)
• Server-side GA4/Google Ads tag (in Server GTM)

✅ Prevents duplicate counting of the same conversion.

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🧪 QA Checklist

Step	Validation
purchase event in success.twig	✅
GA4 tag uses transport_url	✅
Server GTM receives event	✅
GA4 DebugView shows event	✅
Google Ads receives conversion	✅
Values (revenue, ID, currency) match	✅
event_id deduplication logic present	✅

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🧠 Pro Tips

Tip	Benefit
Use first-party subdomain (e.g., gtm.yoursite.com)	Improves cookie lifespan
Send user_id if available	Enables cross-device GA4 tracking
Track refunds similarly	Close the loop for revenue reporting
Log events server-side	Debug attribution gaps
Enable consent mode for GDPR regions	Ensures legal compliance

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📦 Summary Flow

[OpenCart Checkout Success]
        ↓
[PHP → JS dataLayer → Web GTM]
        ↓           ↓
[GA4 Client Tag]   [Server Request to gtm.yoursite.com]
                           ↓
                [Server GTM Container]
                   ↓             ↓
               [GA4]         [Google Ads]

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