This is another bugfix release with only a couple of module-related new features.
The news for this release are:
* “fixed a bug with using the alternate search backend. database search was still done while it shouldn’t.”
* MFH: addon.php allows for addon scripts that are implemented using a module
* Added extra hook ‘posting_custom_action’ to the posting editor code
* MFH: Added signing of editor form data to prevent data tampering
* Added a simple general purpose private key based signature system
* redirect back to vroot if mark-read is used (fixing #403)
For users running 5.1.15 we recommend to replace at least search.php to lessen the load on searching. A couple of smaller fixes were also implemented therefore we recommend every user running phorum 5.1.x to upgrade to this version.
If you want to see the full changelog you can find it here http://www.phorum.org/changelog-5.txt