New mixed Type in PHP 8.0

PHP supports type declarations that can be used for parameters, return values, and properties. However, there may be situations when we need a type that is not supported by PHP yet. We can omit type declaration, but this can cause ambiguous meaning. For example, the developer forgot to specify type declaration, developer omitted type declaration to keep code compatible with old PHP versions, etc.

Since PHP 8.0, we can use a new mixed type. It is equivalent to the following union type:

array|bool|callable|int|float|object|resource|string|null

We can use mixed type anywhere types are accepted: parameters, return values, and properties.

Container.php

<?php

class Container
{
    private mixed $value;
    
    public function getValue(): mixed
    {
        return $this->value;
    }
    
    public function setValue(mixed $value): void
    {
        $this->value = $value;
    }
}

There are some important notes:

• The mixed type accepts any value type, so it cannot be used as part of the union types. For example, string|mixed is illegal.
• When a parameter or property doesn't have a type declaration, PHP assumes that is mixed type.
• When the return value doesn't have a type declaration, PHP assumes that is mixed|void type. However, we cannot specify mixed|void type for return value because mixed type is not allowed as part of the union types.
• The mixed type accepts any value type, including null. So mixed cannot be specified as nullable. It means that ?mixed or mixed|null are not allowed.
• We cannot cast a variable to mixed type. For example, $value = (mixed) $value; emits a parse error.