If the option isn't allowing you to choose it, then it's not available in PHP. I don't know whether that means it's simply not there or the changes haven't be reloaded (you may need to restart your web server/FastCGI server for changes to apply).
You should be able to confirm the PHP values are correct via <url>/admin.php?tools/phpinfo . If they are, you may need to look at web server-based limits (I linked to one related to FastCGI+Apache in the other thread, for example).
if the PHP PECL is installed (and running) you can find it in your PHP info file (yourdomain.com/admin.php?tools/phpinfo). If it's not showing there, you need to restart your HTTP server and PHP processor and look again.