There was a plugin for supporting Xdebug for PHP debugging with kdevplatform 4, now hosted on GitHub: https://github.com/KDE/kdev-xdebug It would be great to bring Xdebug support back to KDevelop 5.
Sounds like a great opportunity to get started with KDevelop development. https://www.kdevelop.org/contribute-kdevelop
It is a pity no one with C++ skills needs this one... I just discovered upgrading from Mageia 5 to 6 that I have lost Kdevelop PHP debug...