I believe this is you misunderstanding the point of the classpath scheme. The classpath scheme is how resource locator will leverage the PSR Path Mapping in "raw" form. The other schemes do not use directly the mappings set forth by something like Composer for the purpose of autoloading.

The classpath should be considered the canonical URI for any file. There is no concept of loading "a class" in the way that your example changes it. Meaning, classpath itself should not be responsible for appending .php to the requested path.