"Poor quality" comes with the territory of popularity. Everyone talks about Java over-engineering horror stories and the Javascript community even coined the "Javascript fatigue" term to summarize the pervasive quality issues in its ecosystem.
But naturally it's perfectly possible to write good quality codebases in those languages.
> like any other random software you'd find on sourceforge or whatever
> could click a button to turn on PHP
> etc.
tl;dr you don't need to know anything. Good summary of why PHP in production "back the days" was usually of very poor quality.