Umm... there isn't an "open_basedir" config directive that I'm aware of, so that's probably why your site is getting bombed when you try to set it. What are you trying to accomplish with this directive? It's quite likely you're thinking of a valid directive, just using the wrong name.
Regarding your syntax question, there are four Apache directives which can be used to set PHP config options in httpd.conf and .htaccess files. They are:
php_flag
php_value
- used to set options in both httpd.conf and .htaccess
php_admin_flag
php_admin_value
- can be used to set options only httpd.conf
More info about these directives is in Ch. 3 of the PHP manual. The syntax for all is as follows:
directive_name <i>name</i> <i>value</i>
HTH.
geoff