[vizeroth]

PSR-2: http://www.php-fig.org/psr/psr-2/

Of course, these have little to do with core PHP, but PSR-1, PSR-0/4, and large portions of PSR-2 have been widely adopted within the PHP development community. The most-debated portion of PSR-2 is, of course, the use of 4 spaces for indentation (no tabs).

PSR-12 aims to bring the guidelines up to date with the latest features in PHP 7, but I've mostly lost track of things in PHP-land since moving to a new job and burying myself in XSL (and occasionally C#).