Error establishing database connection

I’m getting this error when I’m trying to test the connection.
I’m trying to connect an ios app’s database to my website.
Error says: “`Error establishing a database connection
This either means that the username and password information in your wp-config.php file is incorrect or we can’t contact the database server at localhost:3306. This could mean your host’s database server is down.

Are you sure you have the correct username and password?
Are you sure you have typed the correct hostname?
Are you sure the database server is running?`”