hi,
open_basedir is not actually for specifying the tmp dir that php will use. it is a security restriction used to restrict what certain php functions can access.
http://www.php.net/features.safe-mode#ini.open-basedir
If this is a shared server than you will be causing bigger security problems than exist by default with php on plesk[1] by removing the open_basedir restriction. If you must, howerver, you would need to do this in a vhost.conf file for the site rather than the httpd.include file as plesk will overwrite httpd.include. Please detects a vhost.conf and/or vhost_ssh.conf file and makes an include to it from the site's httpd.include.
In that vhost.conf (a file you have to create in the conf/ directory for the site) you would need to remove the open_basedir restriction in a different context than just in the VirtualHost definition, as one is already supplied there by plesk (in the httpd.include). So you would use something like this in your vhost.conf:
CODE
<Location />
php_admin_value open_basedir "list_of_accessible_directories_here"
</Location>
or
CODE
<Location />
php_admin_value open_basedir none
</Location>
to remove it altogether. Then run:
CODE
/usr/local/psa/admin/bin/websrvmng --reconfigure-vhost --vhost-name=yourdomain.com
apachectl configtest
apachectl graceful
(only run apachectl graceful if the configtest is OK).
Anyways, that's the information you asked for, but
please note that disabling open_basedir is Not A Good Idea unless you really know what you're doing, so I'd suggest instead trying to figure out how to tweak the open_basedir value rather than get rid of it completely.
Notes:
1. Just about all control panels have very insecure default site script security setups. Ensim is the only one that allows for very good securty for a total novice, but it's a horrible control panel in just about every other aspect. One's best bet is learning how to make plesk secure.