# Warning: Undefined variable $x

**Code:** `Undefined variable` · **Category:** PHP · **Seen on:** PHP

PHP read a variable that was never assigned in the current scope.

## Meaning

In PHP 8 reading an unset variable raises a warning (a notice in PHP 7) and the value is treated as null. Functions have their own scope, so variables from outside are not visible unless passed in, captured with `use`, or declared global.

## Common causes

- Typo in the variable name
- Variable only set inside an if branch that didn’t run
- Using an outer variable inside a function or closure without `use`
- Template expecting a variable the controller never passed

## Quick fix

1. Initialise the variable before any branch
2. Pass values into functions as parameters
3. Capture outer variables in closures with `use ($var)` or arrow functions
4. Use `$x ?? 'default'` for optional values

## Detailed fix by platform

### PHP

1. Turn on `error_reporting(E_ALL)` in development so every undefined variable is visible
2. Run a static analyser (PHPStan/Psalm) to catch them before runtime

## Code examples

### Closure scope

```php
$discount = 0.1;
$prices = array_map(function ($p) { return $p * (1 - $discount); }, $prices);       // ❌ undefined
$prices = array_map(fn($p) => $p * (1 - $discount), $prices);                       // ✅ arrow fn captures
```

## How to diagnose

1. **Name** — Is the spelling identical?
2. **Scope** — Is it defined in this function?
3. **Flow** — Is it assigned on every path?

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Source: [WhatIsMyError](https://whatismyerror.com/errors/php/undefined-variable)