RESOLUTION
This article describes versions of PHP that can be used in Plesk and recommended configuration of php.ini files.There are two versions of PHP included to and supported by Plesk: PHP4 and PHP5. PHP can work in two modes: as CGI or as ISAPI. Both versions of PHP use php.ini files, that are different depending on version of PHP.
Default location of php.ini for PHP4 is Windows base directory %systemroot%.
Default location of php.ini for PHP5 is PleskPHP5 base directory %plesk_dir%Additional\PleskPHP5
A custom php.ini with PHP4 in CGI mode can be used. In order to apply that, the php.ini file needs to be place into the /httpdocs folder.
But there is no such option with PHP4 in ISAPI mode which always uses the default php.ini of PHP4. Also it is impossible with PHP5 in either modes, where default php.ini of PHP5 is used.
The default location of php.ini file in PHP5 is defined by the following value in the registry:
HKEY_LOCAL_MACHINE\SOFTWARE\PHP\5\IniFilePath
Additional information
Consult with the article for more information on differences between ISAPI and CGI modes:http://msdn2.microsoft.com/en-us/library/ms525172.aspx
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