Great question! While __DIR__ is generally a reliable way to dodge relative path pitfalls (like broken includes when the working directory changes), it does have a few notable drawbacks and edge cases to keep in mind:

• Bypasses PHP's include_path lookup system
  When you use __DIR__ . '/path/to/file.php', you’re forcing an absolute path lookup—PHP won’t check any directories listed in include_path for the target file. If your project relies on include_path to load shared libraries (like legacy PEAR packages or reusable utility files), this approach breaks that flexibility. You’ll have to hardcode full paths instead of leveraging the standardized, centralized lookup system.

• Symbol link path mismatches
  __DIR__ returns the actual filesystem path of the current script, not the path of any symbolic link pointing to it. If your deployment uses symlinks (e.g., linking a development directory to a web server’s document root), this can lead to unexpected missing files. For example: if /var/app/src/script.php is symlinked to /var/www/script.php, __DIR__ will return /var/app/src, so __DIR__ . '/lib.php' will look in /var/app/src even if your lib.php lives in /var/www alongside the symlink.

• Tighter coupling to directory structure
  Hardcoding paths with __DIR__ locks your code to your project’s current folder layout. If you rearrange directories (e.g., moving a module from src/ to src/modules/), every __DIR__-based path pointing to or from that module will need manual updates. Relative paths, by contrast, can stay valid as long as the relative hierarchy between files remains unchanged.

• Redundancy with modern autoloaders
  If you’re using a modern autoloader (like Composer’s PSR-4 implementation), manually using __DIR__ to include files is often unnecessary and redundant. Autoloaders handle path resolution automatically, and mixing manual require calls with autoloading can lead to duplicate file includes or confusion about how dependencies are loaded.

• Minor readability overhead
  For simple, nearby file includes (e.g., a config file in the same directory), require './config.php' is more concise and readable than __DIR__ . '/config.php'. Overusing __DIR__ can clutter your code with repetitive boilerplate, especially in projects with many nested includes.

It’s worth emphasizing that these downsides are contextual—__DIR__ is still the best practice in most cases to avoid the far more common pitfalls of relative paths. But being aware of these edge cases helps you pick the right approach for your specific project setup.

内容的提问来源于stack exchange，提问作者gnusey