If you want to use the DOM Parser in bigger projects you "polute" the global namespace with the defines and the helper functions from the DOM parser.
Although it is a breaking change, I strongly recommend to move the defines and helper functions declared outside of the two classes to a static helper class.
The first release candidate for version 2.0 allows you to manage the parser inside namespaced projects: https://sourceforge.net/p/simplehtmldom/news/2019/10/php-simple-html-dom-parser-20-release-candidate-1/
Please don't hesitate to file bug reports if anything is missing.