I tried it 2 years ago.
It did about 80% of what was need to be done.
And the remaining 20% was a pain in the butt since the ASP->PHP was not the cleanest code, so when I fixed the remaining code I had to get my head around the original crap ASP turned into crap PHP.
If it is not 100% seemless, EVERY TIME, I can't let him use it. He'll screw up and I'll take the fall.