Undefined

Something undefined has not been defined yet. The definition may vary from structure to structure: a variable is created upon its first writing, while a class needs a declaration.

There are many error messages, that are raised when attempting to use an undefined structure. The reasons include typos, missing compiled or custom code, order of execution.

<?php

// class x is defined, or declared
class x {}

// variable $y is undefined
echo $y;

?>

See also https://hbgl.dev/php-now-has-support-for-undefined/