My primal specialization is PHP. I build WordPress plugins and have 4+ years of experience in it. During this period I got good understanding of how this platform work under the hood.
The most complicated plugin that I develop named Network Posts Extended. It has about 80 different attributes. It is made to be able to display posts from the blogs of the Multisite network. The most challenging work was building Plugin Manager. It controls what users can do with other plugins in the admin area, i.e. activate/deactivate or manage settings based on their roles. It was hard to find what hooks can be used. I had to dig dive into the WordPress API.