I had a gut-feeling using slots to quickly switch between different versions of PHP in Gentoo would be straightforward and it was.  The only piece of the puzzle that eluded me for a while, in the end, was working out how to tell portage to install a specific version. This is acheieved with an equals sign. As such: (Assuming in this instance 5.2 is already installed.)

# emerge \=dev-lang/php-5.3.3-r3

Then to switch versions:

# eselect php set apache2 X
# /etc/init.d/apache2 restart

Note: To display available versions when 'eselecting' use:

# eselect php list apache2