Strictly, to support SO_REUSEPORT need linux kernel >=3.9

Worker.php

@@ -2216,6 +2216,7 @@ class Worker
         // Turn reusePort on.
         if (static::$_OS === \OS_TYPE_LINUX  // if linux
             && \version_compare(\PHP_VERSION,'7.0.0', 'ge') // if php >= 7.0.0
+			&& \version_compare(php_uname('r'), '3.9', 'ge') // if kernel >=3.9
             && \strtolower(\php_uname('s')) !== 'darwin' // if not Mac OS
             && strpos($socket_name,'unix') !== 0) { // if not unix socket