What do 'Undefined array key' and 'Undefined variable' warnings mean in PHP 8 and how do I fix them?

PHP 8 promoted these from notices to warnings (and "Undefined offset/index" was unified to "Undefined array key"). The code was always buggy; PHP now tells you. Fix the cause instead of lowering error reporting.

Undefined array key

// before
$name = $_POST['name'];

// null coalescing: default when missing or null
$name = $_POST['name'] ?? '';

// check explicitly
if (array_key_exists('name', $row)) {}
if (isset($row['name'])) {}          // false if the key exists but is null

??= assigns a default when missing: $config['debug'] ??= false;.

Undefined variable

$total = 0;                  // initialise before loops and conditionals
foreach ($items as $i) { $total += $i['price']; }

Typical cause: a variable assigned only inside an if branch, or a typo. Use static analysis: phpstan analyse -l 5 src finds them all.

Related PHP 8 changes you may meet when upgrading

• Attempt to read property "x" on null: use the nullsafe operator $user?->profile?->name.
• Deprecated: Creation of dynamic property (PHP 8.2): declare the property in the class, use #[AllowDynamicProperties] only as a stop-gap, or stdClass.
• Passing null to non-nullable internal function parameters is deprecated in 8.1 (strlen(null), trim(null)); cast or default: trim($s ?? '').
• ${var} string interpolation is deprecated in 8.2; use {$var} or $var.
• Implicit float to int loss in array keys is deprecated in 8.1.

Do not use @ or error_reporting(0)

The @ suppression operator hides real errors and is slow. Keep E_ALL in development and log in production (see the PHP error reporting topic).

Tools for large upgrades: Rector with the PHP 8 set rules, php -l for syntax, and PHPCompatibility for PHP_CodeSniffer. Run your tests on both versions during the migration.