If horde webmail is not working on windows + plesk server and receiving following errors when login to horde….

Warning: main() [function.include]: Failed opening ‘PEAR.php’ for inclusion (include_path=’C:\Inetpub\vhosts\webmail\horde\lib;.;./includes;./pear’) in C:\Inetpub\vhosts\webmail\horde\lib\core.php on line 46

Warning: main(): Failed opening ‘PEAR.php’ for inclusion (include_path=’C:\Inetpub\vhosts\webmail\horde\lib;.;./includes;./pear’) in C:\Inetpub\vhosts\webmail\horde\lib\core.php on line 26

And so on………

This happens mainly because PHP and Zend installers force PHP to use a default configuration file for all applications, but horde uses a special configuration file which is placed to its folder.

Solution:
Please edit registry by command regedit and clean up the default configuration location by removing the following key:

HKEY_LOCAL_MACHINE\SOFTWARE\php\IniFilePath