Fix previousPath() returning full URL for external referrers (fixes laravel/framework#57456)

[JoshSalway]

Summary

Fixes laravel#57456 -- previousPath() returns the full URL path from external referrers instead of just the path portion relative to the application. When a user arrives from https://google.com/search?q=laravel, previousPath() returns /search instead of / (no meaningful previous path within the app).

Root Cause

The bug exists because previousPath() uses string subtraction (str_replace($this->to('/'), '', ...)) to strip the application's base URL from the referrer URL. When the referrer is an external domain (e.g., https://google.com/search), str_replace finds no match and returns the full external path /search unchanged. The method assumes the referrer always comes from the same domain, which is incorrect for external referrers, bookmarks, and cross-origin links.

Additionally, the str_replace approach fails when the application is mounted at a sub-path (e.g., https://example.com/subdir/) because it performs a literal string replacement that can't properly handle base path stripping.

Why This Fix Works

This fix works because it uses parse_url($url, PHP_URL_PATH) to extract only the path component from the referrer URL, completely ignoring the domain and query string. For base path stripping, it uses parse_url($this->to('/'), PHP_URL_PATH) to get the application's base path and removes it with a regex anchor. This approach is domain-agnostic -- it correctly extracts the path regardless of whether the referrer is internal or external. External referrer paths are returned as-is, which is the correct behavior (the path is meaningful to the caller).

Alternatives Considered

We chose this approach over comparing domains and returning / for external referrers because: (1) the method's contract is to return a path, not to validate origin; (2) some applications legitimately use previousPath() to inspect where users came from, even external paths; (3) the parse_url approach is simpler, handles edge cases (missing paths, query strings, fragments) natively, and correctly handles sub-path deployments that the old str_replace approach broke.

Files Changed

• src/Illuminate/Routing/UrlGenerator.php -- Rewrote previousPath() to use parse_url() instead of str_replace()
• tests/Routing/RoutingUrlGeneratorTest.php -- Added tests for external referrers and sub-path deployments