A6664c26c66d0c39fbedb80912eed80是一个有效的PHP类名吗?

I'm using codeigniter, and I'd like to use a string such as A6664c26c66d0c39fbedb80912eed80 for my class name. However I just get a 404 for it.

The PHP docs state:

The class name can be any valid label which is a not a PHP reserved word. A valid class name starts with a letter or underscore, followed by any number of letters, numbers, or underscores.

http://php.net/manual/en/language.oop5.basic.php

which means it should be alright. Any idea what I'm doing wrong?

You wouldn't be getting a 404 from a PHP syntax / compile error. You're probably not running this code you think you are. Echo something directly before and after to test.

well, the class name should be alright, but you should use a readable name. Besides that, you should check whether the webserver can access the file (check file permissions).