I hope this makes sense - it's how I thought about it a millennia ago when I was learning php - you can mix php and html, but you can't combine them.
In other words, as Barand said above, if you want to output html without using the print or echo functionality of php, you need to exit php mode and enter the browser's default mode of html output by using the '?>' php closing tag. By the same regard, if you intend to inject php into your html you need to specifically enter php mode using the '<?php' opening tag.
There are obviously exceptions to this (certain templating languages, etc.) but these are things to think about later on - right now don't worry about it. Just make sure all of the php code you're executing within html is contained in the '<?php ?>' tags.