Single backslash triggers autoloading with empty string

[istepaniuk]

Description

The following code:

<?php

spl_autoload_register(function ($class_name) {
    var_dump($class_name);
});

class_exists('\\');

Resulted in this output:

string(0) ""

But I expected NO output instead.

This seems unexpected. Calling class_exists(''); does not trigger the autoload callback, but a single backslash in the class name does (as does invoking method_exists('\\', 'test'), etc.)

I might be wrong but I can't think of a reason why to invoke the callback in this case, in fact, composer fails because it assumes that $class_name is never empty.

PHP Version

8.2 (and others)

Operating System

Debian 11

[damianwadley]

It also attempts autoloading for obviously invalid class names; the only edge case it actually tests for is an empty string, even though it does later trim a leading \. The order of operations there could be reversed, but:

IIRC last time this subject came up, the consensus was that it wasn't necessary to try to validate the class name beforehand because nobody should be attempting to test for/autoload a class with an invalid name - doing so means there's buggy or possibly even dangerous code involved.

[Girgias]

You can also try to autoload a class name which starts with a digit, something which is not possible to define either.

[istepaniuk]

I think it makes perfect sense that there is no validation, but perhaps we shouldn't autoload if we end up with an empty string, the same way we don't do it for actually empty strings.

I report this because it seems to be unexpected behavior and I imagine it's caused by checking for empty before trimming, but I reckon it's quite an edge case.

[KapitanOczywisty]

You can also try to autoload a class name which starts with a digit, something which is not possible to define either.

To be noted: class_alias allows to define all sorts of invalid names, even \ https://3v4l.org/1lYME

[iluuu1994]

I don't think it makes sense to change behavior here. I'm moving this to documentation to see if it can be improved in any way.