PER-CS 3.0

'<?php' opening tag should be lowercase  

Reports occurrences of the <?php opening tag that are not written entirely in lowercase.

'exit'/'die' should be called with parentheses  

Reports usages of exit or die without parentheses.

'set' hook on promoted property must use shorthand form  

Reports long-form set hooks on constructor-promoted properties.

Acronym should have only the first letter uppercase  

Reports class, method, property, function, parameter, and local-variable names where an acronym is written in all uppercase letters (e.g.

Heredoc can be nowdoc  

Reports heredoc string literals that contain no variable interpolation, no expression interpolation, and no escape sequences, and could therefore be written as nowdoc.