This has been an issue since day 1 on our server which is using IIS 6.
After several changes in php.ini file, I've tried to check phpinfo() and found out that the changes that I've made did not reflect. So I've tried a lot of things such as restarting the website etc... but only 1 simple solution resolves the problem.
The simplest solution is to recycle the application pool where the website resides.
But always take note that it is highly advisable to do this during maintenance period or when no one is using the application to avoid interruption.