Undefined array key 🐘 PHP

Warning: Undefined array key / Undefined index

Your code reads an array key that doesn’t exist — common with $_GET, $_POST and decoded JSON.

Seen on: PHP

Meaning

PHP 8 reports “Undefined array key”; PHP 7 said “Notice: Undefined index”. It’s a warning, not fatal, but the value is null and often breaks logic downstream — and printed warnings can corrupt JSON or headers.

Common causes

  • Form field not submitted (unchecked checkbox, wrong name)
  • Query parameter missing
  • API response shape differs from expectations
  • Typos in key names

⚡ Quick fix

  1. Use the null coalescing operator: $_GET['page'] ?? 1
  2. Check with isset() / array_key_exists()
  3. Validate input up front

Detailed fix by platform

PHP

  1. Defaults with ??:
    php
    $page  = (int) ($_GET['page'] ?? 1);
$email = trim($_POST['email'] ?? '');

Code examples

Reading optional input safely

php
// ❌ Warning: Undefined array key "newsletter" when the checkbox is unchecked
$subscribe = $_POST['newsletter'];

// ✅
$subscribe = isset($_POST['newsletter']);
$page      = max(1, (int) ($_GET['page'] ?? 1));
$sort      = in_array($_GET['sort'] ?? '', ['name', 'date'], true) ? $_GET['sort'] : 'date';

Nested JSON with defaults

php
$data = json_decode($body, true) ?? [];
$city = $data['user']['address']['city'] ?? 'Unknown';   // ?? short-circuits the whole chain

How to diagnose

  1. Key — Which key and which line (from the warning)?
  2. Source — Is the field actually sent? Check the form name / query string / JSON
  3. Shape — var_dump() the array — is the structure what you expect?
  4. Guard — Use ?? defaults or isset() for optional keys

🧠 Still stuck? Analyze your error

Paste the full message, response headers or stack trace — we'll detect the platform and point to the most likely cause.