I think that if PHP reports that dbmopen() is not a function then your version of PHP was not compiled with the --with-db option. This will include all the dbm functions, otherwise you can't use them.
To find out if you have this option include this in a php document:
<?php
phpinfo();
?>
and open it in a browser. Near the top should be an item 'Configure Command'. Next to it it should list what PHP was configured with. If '--with-db' doesn't show up you can use dbmopen() until you recompile using the '--with-db' option.
PHP talks about it here:
http://www.php.net/manual/en/ref.dbm.php
If you didn't compile your PHP yourself, and don't want to learn how, you are going to have to find a PHP package someone has built that was compiled using this parameter.
This site has some great comments by people where you can find good pre-compiled versions of PHP. Just do a search for '--with-db' and possibly whatever OS/Webserver you are using (IIS/Win?). If you can't find a package, compiling the source really isn't so bad, but you'll need a C compiler. I dont think windows comes with one, but Linux/Solaris should.
Hope this helps
Good luck