Recompile php with mcrypt
PhpMyAdmin (available at SlackBuilds.org) works well when you compile PHP with the mcrypt extension. After installing PhpMyAdmin, you may see a warning that it cannot find the mcrypt extension. Unfortunately the official Slackware package does not include this extension. Recompiling PHP is necessary, as is enabling the mcrypt.so extension in /etc/httpd/php.ini. The first step is a little tricky, since compiling PHP requires a little trick: you must compile alpine during the PHP build. It is fairly simple once you have things in place, but this tripped me up when I first tried to recompile PHP.
Install mcrypt and libmcrypt
mcrypt is available from SlackBuilds.org. First build and install mcrypt and libmcrypt before compiling PHP.
Go to a Slackware mirror and download Pat's Slackbuild directory for php from the source/n directory. I used
# wget --mirror <mirror>/n/php
to download the whole directory. You will also need to download the alpine/ directory into the parent of the newly-created php/. In other words, you should have a parent directory with both php and alpine. For example, mine is ~/SlackBuilds/n, where I have two subdirectories: one for alpine/ and one for php/. An alternative to wget is to use a file manager capable of ftp with drag and drop, such as Nautilus.
Be sure to chmod +x both the alpine and php SlackBuilds before attempting to build. If you don't, things may go for quite a while depending on the speed of the machine you're building on before you get an error (and you will get an error).
The official PHP SlackBuild includes a configure flag for mcrypt, so there is no need to edit the SlackBuild. You may want to specify a BUILD variable on the command line, so the package does not appear to be official (it is not official):
# BUILD=3_<your initials> sudo ./php.SlackBuild
Install the new package with upgradepkg. Keep in mind that you will need to repeat this process whenever there is a security alert for php. In that case, download the new source, update the VERSION variable, recompile and upgrade. For this reason, I've kept ~/SlackBuilds/n the way it is with alpine and php subdirectories.
Configuring PHP to use mcrypt is easy: add the line
to /etc/httpd/php.ini and then restart apache. Now when you log in to PhpMyAdmin, you should no longer see the warning about mcrypt.