I have downloaded aps-php-runtime-2.1-283.zip library and documentation says unzip it and place under default include_path directory. I checked in php.ini file but it shows default location as .:/php/includes. What is the actual directory that I have to place third party libraries to use in global as follows.
<php?
require_once "aps/2/runtime.php";
?>
The default directory is indeed /php/includes, and it does not exist on a Mac. You have to create it if you wish to use the default one.
But changing include_path and making it point to a different directory is also a perfectly valid approach.