Undefined Array Key / Offset Errors

TL;DR

Explanation

PHP 7 emits E_NOTICE for undefined offset. PHP 8 emits E_WARNING. Neither throws — the expression evaluates to null. Undefined index means string key missing, undefined offset means integer key missing. Best practices: use isset($arr['key']) to check existence, $arr['key'] ?? 'default' for null-coalescing default, array_key_exists() when null is a valid value (isset() returns false for null values). In typed contexts (PHP 8), use dedicated data structures or value objects instead of raw arrays to prevent key access errors.

Common Misconception

Why It Matters

Common Mistakes

• Using $arr['key'] without isset() check — produces warning and null.
• Confusing isset() with array_key_exists() when null is a valid array value.
• Not enabling E_WARNING in development — errors become invisible.

Code Examples

$config = ['debug' => true];
echo $config['database']; // Warning: Undefined array key 'database'

// Null coalescing:
$db = $config['database'] ?? 'mysql://localhost/app';

// When null is a valid value:
if (array_key_exists('database', $config)) {
    $db = $config['database']; // could be null
}

// PHP 8.1+ — first-class enums/value objects are better than raw arrays

References

• ↗ https://www.php.net/manual/en/function.array-key-exists.php

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