class_exists() with autoload:true (the default) will load the class. If the class declaration doesn't parse, you get an error, as usual.

Incidentally, the reproduce code gives a different error https://3v4l.org/oEW4q:

Fatal error: Uncaught Error: Undefined constant "PromoteConstructorProperty" in /in/oEW4q:11

class_exists(PromoteConstructorProperty); 

should probably be:

class_exists(PromoteConstructorProperty::class);

That makes sense. But to put it another way for clarity - calling class_exists() on a class that is not correctly defined for the PHP version used a runtime cannot be done?

If that is the case - should we change the docs to reflect this? 

"class_exists — Checks if the class has been defined" -> https://www.php.net/manual/en/function.class-exists.php

I'll start a thread in internals (to further discuss options) if the above is true as it would seem that one can no longer use class_exists() in code bases that support multiple PHP versions. Granted I may be missing something obvious :)

https://3v4l.org/oEW4q even

Good catch dan!

<?php

class PromoteConstructorProperty
{
    public function __construct(
        public string $var
    ) {
    }
}

$bool = class_exists(PromoteConstructorProperty::class);

-> https://3v4l.org/4cMW4

oops..

Property promotion is implemented since PHP 8.0 only. It is totally expected that you get a parse error in PHP 7.