How efficient is define in PHP?

define("SETTING", 0);
$something = SETTING;

$setting = 0;
$something = $setting;

php > $cat='';$f=microtime(1);$s='cowcow45';$i=9000;while ($i--){$cat.='plip'.$s.'cow';}echo microtime(1)-$f."n";

php > $cat='';$f=microtime(1);define('s','cowcow45');$i=9000;while ($i--){$cat.='plip'.s.'cow';}echo microtime(1)-$f."n";