It doesn't work with PURL

OK...why? I don't use PURL so someone would have to help figure out why.

I've tried debugging this a little, and hit the same problem. It seems that in url_alter's custom_url_rewrite_inbound(), any other modules which may depend on it are not fully loaded yet. Thus the result of module_implements('url_inbound_alter') (at least the first time) returns an array with only 'url_alter' in it. This result is statically cached in the module_implements function.

Next, every call to module_implements('url_inbound_alter') will return exactly the same value, omitting any plugins you may actually have, because they were not loaded the first time the custom_url_rewrite_inbound() was called.

What works for me, although it is not a very clean solution I guess, is adding these three lines of code to purl.module, at line 161 (after the if (!function_exists('custom_url_rewrite_inbound')) block):

else {
  module_implements(NULL, FALSE, TRUE);
}

With that bit of code, the static cache is cleared when purl is loaded. The next time custom_url_rewrite_inbound() is called, the purl implementation of the hook is properly discovered.

As shown in the documentation, modules that implement hook_url_inbound_alter() also need to have hook_boot() or hook_exit() to ensure they're loaded by Drupal before the full bootstrap happens.

Adding an empty hook_boot() to purl.module indeed seems to fix the problem as well (after clearing caches etc.).

Moving this one over to the Persistent URL module.

Confirmed and committed: http://drupal.org/cvs?commit=358682

Thank you!

This is no longer a requirement for url_inbound alter. However purl needs to hang on to the unmodified querystring from before the first drupal_path_initialize() was run.