The initial interview was an informal chat about past projects I've worked on. The next step was to write some code for a test WP plugin to improve its performance and answer a set of questions regarding the plugin. Although the code I wrote objectively improved performance and met the other technical criteria, I feel like some of my other answers regarding minor issues were nit-picked.
After a little research, I found a blog article by one of their engineers who says they are seeking to hire "women and non-binary people (trans/cis/gnc)", in her words, and "we reviewed our hiring process, we realized that the demographics of people we attract to apply are not in line with the demographics of the people we hope to hire". Since I don't fit that description, and they knew that during the interview process, it makes sense that minor, subjective issues would be raised with my work. I can't post the link to the engineer's blog because of Glassdoor's policy. It shouldn't be difficult to search the internet for it, though.
Bottom line: If you don't fit into any of the categories they list, don't be surprised if you don't get hired.
Questions ranged from things like asking how many password combinations could be generated with certain parameters to open-ended questions about code complexity.
The following metrics were computed from 2 interview experiences for the Automattic PHP Developer role in United States.
Automattic's interview process for their PHP Developer roles in the United States is extremely selective, failing the vast majority of engineers.
Candidates reported having very negative feelings for Automattic's PHP Developer interview process in United States.