PHP on Windows loads its configuration file php.ini during the startup of the process php-cgi.exe . When PHP is run on IIS via FastCGI the php-cgi.exe processes are re-used to handle many requests. If configuration settings in php.ini file get updated
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Read the complete post at http://blogs.iis.net/ruslany/archive/2010/03/30/php-on-iis-monitorchangesto-setting-in-fastcgi.aspx