To improve performance in cases where Drupal is trying to find a file that does not exist in the filesystem, there have been some changes to the drupal_get_filename() API function.
For site builders
When Drupal cannot find a file that it expects to be in the filesystem, it will no longer continually search for it on a large number of page requests (which can hurt your site's performance). Instead, it will record a PHP warning informing you of the problem and pointing you to additional documentation about the issue.
An unexpected missing file usually indicates a problem that either developers or site builders need to solve, so this warning should generally not be ignored.
Site builders are reminded that on production sites, it is always recommended to configure the site not to display errors and warnings to the screen. (This configuration will prevent end users from seeing the above warnings, although they will still appear in the site's logs where administrators can view them.)
For developers
The drupal_get_filename() API function now has a new $trigger_error parameter, which determines whether a PHP warning should be triggered if the file being searched for is missing or moved in the filesystem.
The default value is TRUE since (as noted above) an unexpected missing file usually indicates a problem that either developers or site builders need to solve, but you should set this to FALSE if you are deliberately using this function to determine whether a module or theme is missing (this is an unusual use case). Code which previously called drupal_get_path(), drupal_get_filename() or similar functions for this particular purpose should switch to using drupal_get_filename() with the new $trigger_error parameter instead. For example:
// function drupal_get_filename($type, $name, $filename = NULL, $trigger_error = TRUE);
$module_exists_in_filesystem = (bool) drupal_get_filename('module', 'module_name', NULL, FALSE);
This is an unusual use case, and most existing code is not expected to be affected.
