your original URL is not valid.
URL variables should have a name value pair, for example:
?variable=value
if you have the originbal URL as:
www.mysite.com/myfolder/index.php?myvariable=myfield
that would be more correct. and you could use the following rewrite rule:
RewriteEngine On
RewriteRule ^([^/]*)/$ /myfolder/index.php?myvariable=$1 [L]
I used the mod rewrite generator here to find that:
http://ipdistance.laserprinterdrivers.com/mod_rewrite.php