PHP
Changes to Drupal coding standards are proposed and discussed in issues in the Coding Standards project.
The Drupal coding standards for PHP.
PHP standards are enforced using PHP_CodeSniffer. The rules used are from that project as well as the Drupal coder project and Slevomat.
PHP coding standards
The Drupal Coding Standards apply to code within Drupal and its contributed modules.
API documentation and comment standards
Standards for API documentation blocks in PHP code
API Documentation Examples
This page is intended to be a collection of the complete API documentation examples
Namespaces
Standards having to do with namespaces
Naming standards for services and extending Symfony
Standards for services and Symfony usage
Object-oriented code
Drupal project standards for PHP classes
PHP Exceptions
Basic conventions, exception subclasses, try-catch blocks, inheritance
PSR-4 namespaces and autoloading in Drupal 8
Summary of how Drupal 8 uses the PSR-4 standard for namespace autoloading
Temporary placeholders and delimiters
Standards for what to call delimiters and placeholders in code
Use Drupal Unicode functions for strings
How and why to use Unicode functions for strings
Write E_ALL compliant code
Adjusting the error reporting level
Drupal SimpleTest coding standards
Standards for writing SImpleTest tests prior to Drupal 8