In order to use PHP 4.2 or 4.3 on your RedHat 7.2 system, with mcrypt, you will have to download the PHP 4.1.2 RPM source file, install it, replace the sources of PHP 4.1.2 in the sources directory with the sources of PHP 4.2 (or 4.3...) which you can get from PHP.net, install mcrypt from the source, add the --with-mcrypt directive to the configuration file of PHP, rebuild the RPM (cross your fingers), then install the RPM (cross your fingers again). This is no simple task, and you might have many problems during the building process.
I really suggest you build your PHP from source, and avoid RPMS as much as possible ... well, this is my philosophy, for I have require to have custom modules for my own applications and web site.
Hope this helps, if not, PM me, and we could arrange something.