Fix Call to undefined function json_decode() Error in PrestaShop After Migration

After migrating a PrestaShop website (1.7.x or higher) to a new server, you might encounter this critical error:

bashCopierModifierFatal error: Uncaught Error: Call to undefined function PrestaShop\PrestaShop\Core\Addon\Theme\json_decode()

Don’t worry — this error is usually caused by a PHP JSON extension not being loaded properly, which is required for PrestaShop to function.

📌 Why does this happen?

Even if you’re running the correct PHP version (7.2–8.x for PrestaShop), this issue can occur when:

The JSON extension is not enabled
PHP configuration wasn’t reloaded after a change
The migration skipped or disrupted PHP extension loading

✅ Solutions by Hosting Type

🔹 Case 1: Shared Hosting (OVH, LWS, o2switch, etc.)

Shared hosting doesn’t allow you to restart PHP services directly, but you can force a PHP reload this way:

Step 1 – Change the PHP version in your control panel

Log into your hosting control panel (e.g., OVH Manager)
Go to Hosting > Multisite > Modify configuration
Temporarily switch your PHP version (e.g., from 7.2 to 7.3)
Save, then switch back to your preferred version

🔁 This forces a PHP reload and reactivates extensions like JSON.

Step 2 – Check the website

Reload your site. The error should now be gone.

🔹 Case 2: Dedicated Server or VPS

You have full control here. Just restart the PHP process.

✅ For Apache:

bashCopierModifiersudo systemctl restart apache2

✅ For Nginx + PHP-FPM:

bashCopierModifiersudo systemctl restart php7.2-fpm

Adjust php7.2-fpm to match your PHP version (e.g., php8.1-fpm).

🔍 Check if JSON is enabled:

bashCopierModifierphp -m | grep json

You should see json listed in the output.

🧩 Bonus: Test json_decode Function

Create a test.php file in your site’s root:

phpCopierModifier<?php
echo json_decode('{"test":1}') ? 'JSON OK' : 'JSON ERROR';
?>

Open https://your-site.com/test.php in a browser.

✅ Conclusion

This error might surprise you after a migration, but it’s usually a quick fix. Forcing PHP to reload solves it in most cases, especially on shared hosting.