Mod rewrite

HI

I am just starting out using this and thought I had my head around it so was just trying to do the most basic task but not working and I dont understand why.

.htaccess

RewriteEngine On    # Turn on the rewriting engine
RewriteRule ^items-starting-with-a/$   browse_categories.php?search_string=A    [NC,L]    # Handle requests for "A"

URL
http://domain.com/items-starting-with-a/

Server Error
The requested URL /var/users/userid/domain.com/htdocs/browse_categories.php was not found on this server.

Checked
manually type in domain.com/browse_categories.php and it delivers the page fine as does domain.com/browse_categories.php?search_string=A

Anyone got any ideas where I am heading wrong here
Any help appreciated
Paul