General concepts

PHP tags

The first line of all .php files should be exactly "<?php"
Never use the closing PHP tag (?>)
Never use PHP short tags

Standards

All files must only use UTF-8, without BOM
Always use UTC timezone to store date and time

Side effects

Every .php library file should contain one class exactly
Every .php library file should be side effect free

Naming convention

Singular words (e.g. item, stylesheet, user) should be used throughout files and code instead of plurals
Avoid Hungarian Notation, where file/variable names that indicate their type

Functions

Limit function length
Ensure return points are consistent
A function should only ever do one thing, as indicated by its name
Functions should either mutate or query the system, never both

Variables

Use camelCase for variable naming
Never use global variables
Properties should always be declared
Variables should always be declared at the top of the block they are used in

Open closed principle

Avoid using final keyword unless necessary
Avoid calling functions in global namespace