htaccess rewriterule for query string

I'm new with RewriteRule in .htaccess file.

I'm trying to change the url from:

http://www.example.com/abc.php?val

to:

http://www.example.com/def-ght

I have passed more than 3 days for googling and reading articles about how to write RewriteRule for queury string to redirect to other page. However, I'm still in the block way and can't find out the solution.

Can you guide me to solve this problem, please?