Trouble with SQL statement using variable

I'm having trouble getting an SQL statement to use a variable. Here's
the statement:

        $sql = "SELECT emp_fname FROM employee WHERE username = '$username'";
        $result = pg_exec($conn,$sql);
            if (!$result) {
                exit;
            }
I get the following warning from this:

Warning: Supplied argument is not a valid PostgreSQL link resource in
/var/www/personnel/include/functions.inc on line 25

Is there another way to use a variable in an SQL statement. This format
works with MySQL.

--
Jeff Self
Information Technology Analyst
Department of Personnel
City of Newport News
2400 Washington Ave.
Newport News, VA 23607
757-926-6930