Fatal error: Declaration of Foo::__toString(): void must be compatible with Stringable::__toString(): string

After upgrading to PHP 8 I’m now seeing this Fatal error what’s wrong with my code ? And what is this Stringable?

Answer

Prior to PHP 8 it was possible for a programmer to write magic methods that have signatures that don’t match the internal signature of them.

So for example

class Foo {
    public function __toString(): void {
    
    }
}

Although the return type void doesn’t match the method’s documentation

public __toString ( void ) : string

PHP was not complaining!

However, with the advent of PHP 8 the signature of a magic method must follow that magic method’s documentation.

public function __toString(): string {

}

Otherwise you’ll see a Fatal error!

Note: The __construct() and __destruct() are excluded from this new change, since there is not concept of them returning values in almost every computer language.

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What is this Stringable?

Stringable is an interface that is added automatically to classes that implement the __toString() method

See the RFC for more information.