The best minifier for HMTL (and CSS & JavaScript) that I know is the Kangax Minifier: https://kangax.github.io/html-minifier/
Sadly for PHP users, it is written in JavaScript, so although it can be run server-side in Node, it's unavailable for server-side minification in PHP scripts.
I'd thought about porting it over to PHP (the routine can be viewed publicly at https://github.com/kangax/html-minifier), but it would be a mammoth project, something a group of PHP users should take on, not a solitary PHP user.
1) Any interest by anyone in writing a port of it to PHP?
2) What PHP-based HTML/CSS/JS minification routines have you found that work well?
Tom