pg_query won't execute

I'm learning PHP for PostgreSQL and have come across something I can't
figure out.

The statement "SELECT COUNT(*) FROM users WHERE username = 'john' works just
fine in the pgAdmin query tool, but not like this:

$sql = "SELECT COUNT(*) FROM users WHERE username = $username";

If ($result = pg_query($dbconn,$sql)) {

.process statements.

} else {

echo "Could not execute $sql";

}

$dbconn is a valid resource (I've changed up the $sql statement to test, and
it works just fine), so no problem there. The problem is incurred when I
attempt to COUNT something.

Is there a different method I should be using for returning a count?

Thanks

Dale