If Apache won't start after PHP install, check for this scenario:
When I followed the instructions for the Windows installer, using Apache HTTP server 2.2.17, the install completed normally but Apache would then fail to start. It turned out that the installer was not properly configuring the last section of Apache's httpd.conf file:
#BEGIN PHP INSTALLER EDITS - REMOVE ONLY ON UNINSTALL
LoadModule php5_module "c:/php/php5apache2_2.dll"
#END PHP INSTALLER EDITS - REMOVE ONLY ON UNINSTALL
The text above is the correct configuration (in my case, I had asked the installer to place PHP into directory C:/PHP.) However, the installer omitted the string "c:/php/" from the two lines above where you can see them. I had to edit the file, and add that string in manually, before Apache would start correctly.