//first generate a query to get the password from the database
$q = "SELECT password FROM user_information WHERE userid='some_identifier'";
//execute the query
$r = mysql_query($q);
//get the info from mysql
$temp = mysql_fetch_row($r);
//put the password into a variable that is easier to access
$password = $temp[0];
//now that you have the password in that variable, lookup the PHP function mail()
[man]mail[/man] That should get you going.