Single backslash triggers autoloading with empty string

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

Start by reproducing the PHP 8.2 example using spl_autoload_register and class_exists('\'), then compare it with class_exists('') and method_exists('\', 'test'). The issue names no doc-en file or test; determine which PHP documentation entry covers these calls and whether the expected result is a documentation clarification or a runtime bug report.