OpenCart after a PHP 8 upgrade: what breaks

Why it happens

PHP 8 removed deprecated functions and became strict about mistakes PHP 7 silently ignored. Older OpenCart cores and especially third-party modules were written for PHP 5–7 and fail at exactly those spots.

• OpenCart 2.x and early 3.0.x don’t run on PHP 8 without patches;
• in later 3.0.x the core is largely compatible, but modules and themes often aren’t;
• OpenCart 4 targets PHP 8, but 3.x modules don’t work on it.

Typical errors and what they mean

Find the message in the error log (system/storage/logs/error.log or the PHP log in your hosting panel): it names the file and line.

• Call to undefined function each() / create_function() — removed in PHP 8; rewrite with foreach and closures;
• count(): Argument #1 must be of type Countable|array — code counts a variable that is sometimes null; this used to be a warning and is now fatal;
• Undefined array key / Undefined variable — warnings, but with error display on they break the layout and the AJAX cart;
• Required parameter follows optional parameter — outdated parameter order in module functions;
• ionCube Loader … not installed — an encoded module; you need the vendor’s build for your PHP version.

How to upgrade without downtime

• copy the site and database to a staging subdomain;
• switch the copy to PHP 8, log errors to a file and turn on-screen display off;
• walk through the main flow: home, category, filter, product, cart, checkout, payment, admin;
• fix core and module errors, update modules that have PHP 8 releases;
• clear the modification and Twig caches and switch PHP on the live site.

For a typical OpenCart 3 store with 15–30 modules this takes one to a few days of work.

What not to do

Don’t switch the live site to PHP 8 just to see what happens, and don’t “fix” errors by hiding them: warnings disappear, but fatal errors in the cart or payment stay, and you learn about them from lost orders.