Done it already...
Did something very similar for my employer about 4 years ago.... in Access! Not the best solution, but it works...
Tracks regulatory compliance, employee leave/sickness/holidays, appraisals, reporting lines, pretty much anything you can think of from an HR perspective.
Wish i could get £250 per user off them!! Yeah, and pigs might fly!
Must get round to the PHP version! Maybe i could go work for Sage....