I work with development builds and I run multiple NetBeans instances at the same time. I have always one instance running where I develop and a second one for testing purposes. Also I change the NetBeans build almost daily. Fortunately there is the --userdir switch.
This switch has one parameter and this is a path to the folder where NetBeans stores all settings, including information about opened projects. Basically it's something like a workspace. If I need to work on PHP suppport, I start NetBeans with this command
./netbeans.sh --userdir /space/netbeans/userdirs/php
If I need to fix something in Java Web Support I use
./netbeans.sh --userdir /space/netbeans/userdirs/javaweb
and for testing php build
./netbeans.sh --userdir /space/netbeans/userdirs/phptest
Because I use development builds, sometimes the userdir can be damaged and I have to delete it and start NetBeans with a clean userdir, but this is not a big issue. Sure, I can lose all my settings, but I have to be aware that I work with the latest builds and this can happen. On the other hand, I have to delete the userdir very rarely.
I don't use installers, I always download NetBeans as a zip file. These are available on nightly build download page, where OS Independent Zip item in has to be selected in the Platform combobox. There are also really fresh builds publicly available on the continual building machine. The PHP builds are available here. There are available only as zip files. The zip file can be unzipped anywhere on your machine. Then just execute the netbeans.sh or netbeans.exe, which is located in the bin folder, with the --userdir switch.