A new RFC has been finished voting and accepted for PHP - 8. That is "Union Types", which is an extension of PHP's type system.

A union type accepts values of multiple different types, rather than a single one. PHP already supports two special union types:

• Type or null, using the special ?Type syntax.
• array or Traversable, using the special iterable type.

However, arbitrary union types are currently not supported by the language. A Union type essentially declares an OR condition for multiple types in the argument type, return type or property type declaration.

The RFC Proposal:

Union types are specified using the syntax T1|T2|… and can be used in all positions where types are currently accepted (Taken from the RFC example):

class Number {
    private int|float $number;
 
    public function setNumber(int|float $number): void {
        $this->number = $number;
    }
 
    public function getNumber(): int|float {
        return $this->number;
    }
}

Union types support all types currently supported by PHP, with some caveats. This is just a glimpse, to learn more please read the PHP RFC: Union Types 2.0.