PHP_Debug is an open-source project I maintained during several years from 2003 to 2009. It was first released on Sourceforge in 2003; then it became an official PHP Pear package in 2007.
There is no more activity on this project for many years, so it's better to archive it.
You can still install it with PEAR. Use at your own risks!
pear install PHP_Debug
Here are some resources you may find useful: