Merge 'Object-oriented code' page into 'PHP coding standards' page

That sounds like a reasonable plan. I expect the Object-oriented page is newer, maybe added when Drupal 8 was first being developed?

There was also support in Slack for this from larowlan and yesct.

I moved the content, then merged two 'naming' and two 'type hinting' sections. After that I took the liberty of alphabetizing the page by the h2 headings and ensured that all h2 headings had a correct id.

Great job, thanks!
I’d merely suggest renaming the “Naming » Classes” sub-heading → to “Classes, methods and properties”. I was a bit confused by properties missing from the “Naming” sub-menu.