adding of hyperlink of the PHP (#4985)

php/README.md

@@ -17,7 +17,7 @@ generation functionality.
 To use PHP runtime library requires:
 
 - C extension: PHP 5.5, 5.6, or 7.
-- PHP package: PHP 5.5, 5.6 or 7.
+- [PHP package](http://php.net/downloads.php): PHP 5.5, 5.6 or 7.
 
 ## Installation