Class not found when defined after use, if it contains a __toString method

[tobia]

Description

The following code:

<?php

new A();

echo "OK";

class A {
    public function __toString() {
        return "A";
    }
}

Resulted in this output:

Uncaught Error: Class "A" not found

But I expected this output instead:

OK

Here is a simple way to test multiple versions in Bash:
for ver in 7.4.33 8.0.0alpha1 8.2.10 8.2.11RC1 8.3.0RC2; do echo $ver:; docker run --rm php:$ver -r 'new A; echo "OK\n"; class A { function __toString() { return "A"; } }'; echo; done

Result:

7.4.33:
OK

8.0.0alpha1:

Fatal error: Uncaught Error: Class 'A' not found in Command line code:1
Stack trace:
#0 {main}
  thrown in Command line code on line 1

8.2.10:

Fatal error: Uncaught Error: Class "A" not found in Command line code:1
Stack trace:
#0 {main}
  thrown in Command line code on line 1

8.2.11RC1:

Fatal error: Uncaught Error: Class "A" not found in Command line code:1
Stack trace:
#0 {main}
  thrown in Command line code on line 1

8.3.0RC2:

Fatal error: Uncaught Error: Class "A" not found in Command line code:1
Stack trace:
#0 {main}
  thrown in Command line code on line 1

As you can see, this bug was introduced in the 8.x branch, because all 8.x versions are affected, from the first alpha to the latest RCs, including all stable versions I tried. 7.x and earlier are unaffected.

I did not test if other magic methods have the same effect.

PHP Version

8.x

Operating System

No response

[damianwadley]

What changed in PHP 8 was that classes defining __toString would implicitly and automatically implement the Stringable interface. And as happens with other classes that use interfaces, PHP is no longer able to "hoist" the class definition and allow you to use the class before it's defined.

Hoisting is an undocumented courtesy, and official advice is to not make use of it:

Note: Unless autoloading is used, the classes must be defined before they are used.

See also #7873 which has some more information.