What is a PSR (PHP Standards Recommendation)?
Short answer
A PSR (PHP Standards Recommendation) is a specification published by the PHP Framework Interop Group (PHP-FIG) so that PHP libraries and frameworks can work together. PSRs define shared conventions and interfaces, such as autoloading (PSR-4), coding style (PSR-12), logging (PSR-3), HTTP messages (PSR-7) and containers (PSR-11).
The PSRs you will meet most
- PSR-4: autoloading. Maps namespaces to folders, which is what Composer’s
autoloadsection uses. - PSR-12 (now extended by PER Coding Style): code formatting, so code from different teams looks the same.
- PSR-3: a common
LoggerInterface, implemented by Monolog and others. - PSR-7 / PSR-17: HTTP request and response objects and their factories.
- PSR-15: HTTP middleware and request handlers.
- PSR-11: dependency-injection containers.
- PSR-18: HTTP clients.
Why they matter
If your code depends on a PSR interface rather than a specific library, you can swap implementations, say one HTTP client for another, without rewriting your code. That is why framework-agnostic packages type-hint Psr\Log\LoggerInterface instead of a concrete logger.
In practice
Use PSR-4 autoloading through Composer, enforce a coding style automatically with PHP-CS-Fixer or PHP_CodeSniffer, and prefer PSR interfaces at the boundaries of reusable code.
