Q: What is the easiest/fastest way to set up AWS to repeatedly run a small Python or PHP script (24×7)?
Probably the easiest way is to use Lambda. It has a built-in scheduler and you only pay for the time it is running. (Note that in PHP you have to create a lambda package using this tutorial or via node Serverless here.)
If you mean continuously, and not repeatedly, then the caveat for PHP at least, it is a bad idea to have it repeat forever in the process because it was designed to be set up and torn down and might leak memory (with the proper setting to set_time_limit and ignore_user_abort, have it run forever). In those cases you would have something else constantly call the php script via command line and restart.
Other than that, whether PHP or Python or whatever, it is better to create a non-burstable EC2 instance and run it, since it will always have some load. You can write something simple to the crontab that will make sure the process is always running or, better yet, use forever.