Fix "Uncaught TypeError" in PHP 8

PHP Fatal error:  Uncaught TypeError: calculateTotal(): Argument #1 ($items) must be of type array, null given, called in /var/www/html/cart.php on line 14 and defined in /var/www/html/lib.php:5

Common causes

• Passing null where an array or string is expected, often from a failed query or missing input
• Calling count() on null or a scalar, which became a TypeError in PHP 8.0
• Arithmetic on arrays or objects (Unsupported operand types)
• A typed function returning null without a nullable return type such as ?string
• declare(strict_types=1) rejecting a numeric string passed to an int parameter
• Old plugins or libraries not updated for PHP 8

How to fix it

1. Read the message parts. Argument #1 ($items) must be of type array, null given names the parameter and the actual type. 'called in ... line 14' is the caller that passed the bad value; 'defined in' is the function itself.
2. Fix the value at the caller. Find where the variable gets its value and make sure it is the expected type. For example, a database fetch that returns false or null when no rows are found must be handled before passing it on.
3. Add safe defaults. Use count($items ?? []) or (string) ($value ?? '') where an empty value is acceptable. Use is_array() or is_countable() before calling count() on data you do not control.
4. Correct type declarations. If null is a valid value, declare the parameter or return as nullable, such as ?array $items = null or : ?string. Use union types like int|string where both are legitimate.
5. Update outdated code. If the trace points into a plugin, theme or vendor package, update it to a version that supports your PHP version. As a temporary fix, run the site on the older PHP version while you wait for an update.

Defensive fixes

<?php
// Before (TypeError on PHP 8 when $rows is null)
$total = count($rows);

// After
$total = is_countable($rows) ? count($rows) : 0;

// Nullable parameter and return types
function findName(?array $user): ?string {
    return $user['name'] ?? null;
}

How to stop it happening again

• Run static analysis (PHPStan or Psalm) to catch type mismatches before deployment
• Return empty arrays instead of null from functions that return lists
• Test plugins and libraries on your target PHP version before upgrading production
• Validate external input at the boundary and convert it to the expected types