$dbname = "USER";
$sql = "select USERNAME from $dbname";

##</code><code>##

$dbname = "User Names Database";
$sql = "select USERNAME from $dbname";

##</code><code>##

select USERNAME from User Names Database

##</code><code>##

$dbname = "User Names Database";
$tblname = "'" . $dbname . "'";
# or $tblname = '"' . $dbname . '"';
$sql = "select USERNAME from $tblname";

##</code><code>##

select USERNAME from 'User Names Database'