i wonder what you mean by Application Server ...
php will run on most environments. the best environment to choose is a function of what you need your application to do. In the majority of cases a simple linux box will suffice with httpd and mysql installed. you can add further duplicate boxes with a squid server for some type of scalability (with db replication). equally you can also hive off the mysql install to a separate dedicated database server and then also multiply that into load balanced environments. all with open-source software.
some php sites require native microsoft bits to work (for example printing solutions). for this you would need an MS box in your architecture.
some of my apps make very heavy use of external system calls for optimisation. If this is applicable to you, make sure that those command line programmes are available to the platforms you select, preferrably in some nice apt or other package installer.
sometimes your app may be entirely command line, in which case httpd is unlikely to be necessary.
basically your question is similar to 'I need to tie something up, how long should my string be?' We don't know what it is you are trying to tie. If we did we could tell you how long and also whether string is the best medium.