Autoloading does the wiring, and the flushing
With OTEL_PHP_AUTOLOAD_ENABLED set to true, the SDK builds a tracer provider from your environment when the app loads and flushes it when the script ends. That last part matters more in PHP than anywhere else: the request-per-process model means a missed flush is lost telemetry, and autoloading removes that failure mode instead of leaving it to you.