Is it possible to specify multiple return types on PHP 7?

Since PHP 8.0 this is possible.

You can now use union types to specify this:

function test(): Success|Failure
{
    if ($this->condition === false) {
        return new Failure();
    }

    return new Success();
}

The fact that this is possible does not mean that it is always advisable. In many (probably most) situations, using an interface (e.g. Result, which both Failure and Failure would implement) as advised in a different answer, is still much preferable.

But there are other instances where union types could make sense to an alternative to weak typing. E.g. a method that accepts both string and int, or to describe the return type of a function like stripos(), which returns int|false.

As of PHP 8+, you may use union types:

function test(): FailObject|SuccessObject {}

Another way, available in all versions since PHP 4, is for the two objects to share an interface. Example:

interface ReturnInterface {}
class FailObject implements ReturnInterface {}
class SuccessObject implements ReturnInterface {}
function test(): ReturnInterface {}

In this example, ReturnInterface is empty. Its mere presence supports the needed return type declaration.

You could also use a base, possibly abstract, class.

---

To me, for this use case, interfaces are more clear and more extensible than union types. For example, if I later want a WarnObject I need only to define it as extending ReturnInterface -- rather than going through all signatures and updating them to FailObject|SuccessObject|WarnObject.

As noted by bishop, there is an RFC for adding multiple return types. However, I thought I'd add that as of PHP7.1 you can now specify a nullable return type like this:

function exampleFunction(string $input) : ?int
{
    // Do something
}

So this function would take in a string and by adding the question mark before int you are allowing it to return either null or an integer.

Here's a link to the documentation: http://php.net/manual/en/functions.returning-values.php

And here's a quote from that page explaining the usage: PHP 7.1 allows for void and null return types by preceding the type declaration with a ? — (e.g. function canReturnNullorString(): ?string)

Also, here's another thread that relates to this: Nullable return types in PHP7

PHP from 7.2 onward supports the object return type

http://php.net/manual/en/migration72.new-features.php

function test(object $obj) : object
// return any type of object ...