[zelphirkalt]

"Various quality" is a very mild expression for it already.

Add to that the tendencies of many to use PHP directly as a way of cutting pages into non-modular non-composable pieces, that can only be concattenated instead of used everywhere, and add to that a mix of PHP, HTML, CSS and JS in the same file ... well you are set up for a catastrophe in terms of maintainability, which maps closely to how most plugins for WP are written.