You can install two instances of PHP. One php4 and one php5 (one CGI and one apache module). I forgot how it's done, but you should be able to use the apache conf file to assign different users to different instances of PHP.
Thanks, but I don't want to install PHP 4. It's never been installed on this server, and it is now end-of-lifed.
I was hoping to be able to do something like this in a user's .htaccess:
suPHP_Engine on
This didn't work, but I did find something that did work. I used a .conf files for a virtual host and added these lines:
suPHP_Engine on
AddType application/x-httpd-php5 .php5 .php4 .php .php3 .php2 .phtml
suPHP_AddHandler application/x-httpd-php5
I then ran this:
/scripts/ensure_vhost_includes --user=username
It seemed to work fine.