This is a high-level answer as you've not given any specifics.
Reading KML is basically the same as XML (KML being a subset thereof).
Some possible resources:
https://stackoverflow.com/questions/8342337/parse-kml-file-with-php
http://www.boards.ie/vbulletin/showthread.php?p=71913675
http://web.stefanproell.at/wp/2010/01/parsing-kml-files-with-php/
Then you just take the variable that you've read your data to and output it to the database.
