Why is underscore converted to directory separator in the PSR-0 standard?
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The PSR-0 (<https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-0.md>) standard specifies that an underscore in the class name should be converted to a directory separator in the corresponding file name.

To me this does not seem to be a good idea as it creates lots of errors when someone who does not know the standard innocently uses an underscore in the class name and suddenly the autoloader cannot find the class and all sort of weird errors appear (see this stackoverflow issue for example: http://stackoverflow.com/questions/11686922/symfony2-1-mapping-error-class-parents)

So I guess there must be some kind of reason (historical compatibility with some library?) for this "feature". My question is: does anyone know why this was introduced in the PSR-0 standard?

