PHP-FIG's PHP Standard Recommendations (PSRs) define shared conventions so libraries interoperate: PSR-1/PSR-12 cover coding style, PSR-4 covers autoloading, PSR-3 defines a logger interface, and PSR-7 defines HTTP message interfaces. Following these lets frameworks and packages from different vendors work together seamlessly.
Composer's composer.json "autoload" section maps a PSR-4 namespace prefix to a source directory, e.g. mapping "App\\" to "src/".