Hi @ethanisnotdumb, welcome to the Glitch forum!
We don’t have built-in PhpMyAdmin support, and have no current plans to add it. PHP itself isn’t officially supported, although you can get it working by following something like https://glitch.com/~lamp-poc. You might be able to start there and get PhpMyAdmin working in a Remix of that project.
You’re welcome to add a topic in https://support.glitch.com/c/feature-ideas regarding this desire, but as I mentioned it’s not currently on our roadmap, and I wouldn’t expect any significant work towards that support any time in the near future.
As far as other tool options, since your database won’t typically be accessible except from within your project’s container it’s unlikely that you’ll be able to connect an external tool. You might be able to find 3rd party hosting for your database on a platform that offers such tools. You could also take a look at https://glitch.com/@storage for other possibilities.