PHP

Undefined variable / array key (PHP)

You read a variable or array key that was never set.

What it looks like

Warning: Undefined variable $total in /var/www/app/cart.php on line 31
Warning: Undefined array key "page" in /var/www/app/list.php on line 8

What does "Undefined variable / array key (PHP)" mean?

Raised as a warning since PHP 8 (older versions called it a notice). The script keeps running with null in place of the value, which can cause wrong output further on even when the warning itself is hidden.

What causes "Undefined variable / array key (PHP)"?

• A typo in the variable/key name.
• Assuming a request/array key always exists.
• A variable defined only inside an if/loop branch.

How do I confirm the cause?

1. For a variable, check it is assigned on every path — a variable set only inside an if or a loop is undefined when that code is skipped.
2. For an array key, check where the array comes from; request data ($_GET, $_POST) never guarantees a key is present.
3. Look for a typo between where it is set and where it is read ($totals vs $total).

How do I fix "Undefined variable / array key (PHP)"?

• Check the name and that it's set on all paths.
• Use isset()/?? defaults for request data ($_GET['x'] ?? '').
• Initialize the variable before the branch.

How do I stop it happening again?

• Initialise variables before conditional blocks, and read optional keys with ?? defaults ($_GET['page'] ?? 1).
• Show warnings in development (error_reporting(E_ALL)) so they are caught before release.

Got a different error? Paste it into the Explain This Error tool → Identifies the family and the fix — runs locally, nothing uploaded.

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