Have you thought of approaching this from the point of view of 'They learn Perl from you, you learn PHP from them'? I'm not suggesting that you switch over to PHP for work, but it's always good to have other languages under your belt.. also if you do find stuff PHP has that Perl hasn't you can pull it back over our side of the fence
Knowing more languages is never a bad thing, and as you said these two aren't that different so the learning curve shouldn't be too steep.
I suppose the disclaimer here should be that I've not learnt PHP myself as yet, but I think I would do if there was the opportunity to do it in a mutual learning environment such as this could be. I do make a point of continually learning though and when I stall on Perl I generally either pick up one of my other languages for a play, or attack a new one.