Gotchas in Javascript are syntactic/semantic, because there's no standard library worth its name bundled with it.

PHP's gotchas are syntactic/semantic but mostly on its huge standard library, which is an haphazard collection of functions from sending emails to parsing ID3 tags from MP3 files.

EDIT: JS, as the legend says, was written in 10 days, while PHP grew without much long term vision for years.

I think the parent poster was talking about both being messy in the past, but recently having a major version upgrade that deprecated old features. PHP 7 could be compared to ES6 (as well as ES5's strict mode)