Drupal - files[]=lib.php fails to autoload classes of different namespace?

Yes, Drupal 7 knows nothing of namespaces unfortunately (namespaces were only introduced in PHP 5.3 and D7 is compatible with earlier versions than that).

However, life gets infinitely better with the XAutoload module, which is PSR-0 compliant an thus knows all about namespaces.

xautoload is a comprehensive and yet highly efficient PHP class loading suite. Most importantly, it provides Drupal-8-style PSR-0 autoloading for Drupal 7.

The latest release is even moving towards PSR-4 which is great news.

If you install that module you can remove all of the files[] references from your info files, as long as you follow the \Drupal\my_module namespacing pattern. Even if you don't, XAutoload has hooks to let you define your own patterns.

As Clive said above, the Drupal 7 core registry cannot handle namespaces, but [xautoload][https://drupal.org/project/xautoload] is your solution!

The xautoload-7.x-5.x branch has PSR-0 and PSR-4 support, and it can generate classmaps by scanning a directory (and unlike core, this does support namespaces).

Now you have a number of options:

1. Use PSR-4 like in Drupal 8, with your classes in the /src/ directory. See https://www.drupal.org/docs/develop/standards/psr-4-namespaces-and-autoloading-in-drupal-8

2. Put your classes in arbitrary files, and let xautoload scan these files. The result will be cached in the regular Drupal caches, so you need to flush the caches whenever you add a new class file. This is very similar to the Drupal 7 core class registry, but it does support namespaces.

   /**
    * Implements hook_xautoload()
    */
   function MYMODULE_xautoload($api) {
     // Add directories to discover php files with classes.
     // You can be creative with the wildcards.
     // '**' means to scan subdirectories of any depth.
     $api->addClassmapSources(array(
       'includes/**/*.inc'
     ));
   }
   

3. Put your classes in arbitrary files, avoid namespaces, and use the D7 core registry + xautoload wildcards. Put this in your MYMODULE.info file:

   files[] = includes/**/*.inc
   

4. There are some options that are supported for legacy reasons, but these should not be mentioned here.

An alternative is registry_autoload, which uses the regular core registry, but supports namespaces and detects classes automatically.

Advantage of registry_autoload: More light-weight (but does not mean it is faster, the jury is still out on this).

Advantage of xautoload: During development, classes are immediately available without running the registry build.