Mixed

A special type that represents any available type. It is equivalent to not explicitly setting the type, though it is now explicitly done.

A mixed type may be also represented by a union of all possible types.

mixed is useful when literally any type should be supported, such as with a cache system. Yet, it is usually recommended to consider reducing the number of possible types by using a common interface or a union type.

<?php

function cache(string $name, mixed $value) : bool {
     static $cache = [];

     $cache[$name] = $value;

     return true;
}
?>

Documentation

See also Mixed Type PHP 8

Related : Type System

Added in PHP 8.0+