mysql_* to MySQLi

mysql_* to MySQLi

I recently learned that mysql_* has been depreciated and i have a quick
question on how to rewrite something.
$db = mysql_connect("localhost","root","PASSWORD");
if(!$db) die("Error connecting to MySQL database.");
mysql_select_db("FormData" ,$db);
I have tried rewriting it like this...
$mysqli = new mysqli("localhost", "root", "PASSWORD", "FormData", $db);
if(!$db) die("Error connecting to MySQL database.");
But when it posts my form i get the "Error connecting to MySQL database."
error. I was able to fix it by just using this but i wanted to know how to
add in the Error connecting.
$mysqli = new mysqli("localhost", "root", "PASSWORD", "FormData");
Any help would be great as i try to learn all of the new MySQLi stuff!