Add HTML options to a text field in a php file

I have a php script with a text field $_POST['message']. How do I add options to it so that it lets the user format the message as per their needs before sending the message such as making a part of text bold, adding media files (photos), change font size etc?

For now, when the message is sent. There are no html tags. How would you add such an option to a php file? I'm lower than basic so sorry if it's a dumb question.

Thanks for your time :)