I want to let users test out a PHP class of mine, that among other things crops and resizes images.
I want them to write PHP code in a text field, send the form, and then their code will be run. How can I do this?
Or is it other safe ways to let users (anyone) demo a PHP class?
Yes. You can use the eval statement in php (linked earlier by John https://www.php.net/manual/en/function.eval.php), however be very careful. You really don't want users to be able to run code on your machine freely.
My recommendation would try to come up with a different approach to do demos - perhaps a flexible few examples... but don't let them run code directly