As much as I can admire you for trying to compare these: go with what the client wants, it'll be good for you. PHP is able to be run on any OS which has a server written for it and more importantly, the language implementation is free (this could be, through and through, why the client wants use of the language..). MySQL is, again, able to be run on any OS (again, a good choice on their part if they want the code to be run-able on any server they decide to use). ASP.net is primarily class-based, while PHP is the JoaT here in that it has both OOP and Functional Programming models as default ways to go about things. MSSQL can be good at some things, but in my experience, it's expensive and slow in comparison to MySQL when using MySQL's InnoDB storage engine. As previous posters have said, they've each got their ups and downs, therefore neither is better. I uses PHP by preference as I can get more done faster. For an IDE (of sorts), I use PHP.net's documentation and NotePad++ (http://notepad-plus.sourceforge.net/) plus the editor has a built-in help mechanism to do lookups on PHP.net by a key-combination (Alt + F1). Hope you can see where I'm coming from with this as it's not a crime to use free languages and free software, but (imho) it is a crime to get stuck down to just one language pair (ASP.net/MSSQL) just because you cannot see the merits of a different language pair (PHP/MySQL) and accept when a client wants to use them ;).
The worst thing about the darkness is the light at the end - DX-MON