Hi DKB,
If it's your development machine, it is definitely ok to install both platforms: .NET / IIS / MS SQL & PHP / APACHE / MySQL.
If you have already installed .NET as you said, I recommend you to use XAMPP or WAMP for PHP coz all the features are included into one package so that you won't worry about installing every instance separately and setting up the ports to run on...
One thing to note: I have installed in one of my machines both platforms, even some more scripting language platforms such as Perl and other types of languages like ProLog and Oracle 10g Express as DBMS. 90% of the time I spend working on .NET, but, when I have need to do something using PHP / MySQL / Apache, I'm stopping the IIS service by running from cmd: iisreset /stop in order to run Apache server appropriately on the same port that IIS uses.
So, shortly, I keep all services, excapt .NET ones, stopped. I start them when I need to work with them.
So, I would recommend you from my experience not to let all the instances running if you are not working on these in that point of time. So, keep .NET / IIS / MS SQL services running if you are working more with these and stop these of Apache & MySQL (will not eat memory and will not mix up with the other .NET platform services, even though you can avoid this by some manual port settings). Start the services when you will need to work with them...
Hope this helps even I have entered little out of the scop of the question,
Hajan