Warning: file_exist() - open_basedir

Dear Omeka friends,

where can I define an open_basedir?
I have this warning. The Installation was succesful, but i want to check out what this warning means.

Has someone an answer? That would be great!
Thanks a lot for your advice.

Best.
Lea

http://web15.alfa3044.alfahosting-server.de/omeka/install/index.php/index/installed

Warning: file_exists() [function.file-exists]: open_basedir restriction in effect. File(/usr/share/pear/layout.php) is not within the allowed path(s): (/var/www/web15/html/:/var/www/web15/phptmp/:/var/www/web15/files/:/var/www/web15/atd/:/usr/share/php/) in /var/www/web15/html/omeka/application/libraries/Omeka.php on line 23

Warning: file_exists() [function.file-exists]: open_basedir restriction in effect. File(/usr/share/pear/config.php) is not within the allowed path(s): (/var/www/web15/html/:/var/www/web15/phptmp/:/var/www/web15/files/:/var/www/web15/atd/:/usr/share/php/) in /var/www/web15/html/omeka/application/libraries/Omeka.php on line 23

Warning: file_exists() [function.file-exists]: open_basedir restriction in effect. File(/usr/share/pear/logger.php) is not within the allowed path(s): (/var/www/web15/html/:/var/www/web15/phptmp/:/var/www/web15/files/:/var/www/web15/atd/:/usr/share/php/) in /var/www/web15/html/omeka/application/libraries/Omeka.php on line 23

Warning: file_exists() [function.file-exists]: open_basedir restriction in effect. File(/usr/share/pear/view.php) is not within the allowed path(s): (/var/www/web15/html/:/var/www/web15/phptmp/:/var/www/web15/files/:/var/www/web15/atd/:/usr/share/php/) in /var/www/web15/html/omeka/application/libraries/Omeka.php on line 23