ho usato questo e funziona; prima mancava la formattazione giusta

function diff_microtime($mt_old,$mt_new)
{
list($old_usec, $old_sec) = explode(' ',$mt_old);
list($new_usec, $new_sec) = explode(' ',$mt_new);
$old_mt = ((float)$old_usec + (float)$old_sec);
$new_mt = ((float)$new_usec + (float)$new_sec);
return $new_mt - $old_mt;

}

$_start = microtime();
/// qui eseguo la select
$time2=number_format(diff_microtime($_start,microt ime()), 4);