$file = $_SERVER['DOCUMENT_ROOT'].'/'.$domaindir.'/test.php';

$dbfile = "include('".$_SERVER['DOCUMENT_ROOT']."/".$domaindir."/web_".$dbname.".inc.php');";

$file       = file('yourfile.php');

$first_line = array_shift($file);       // Remove first line and save it
array_unshift($file, $yourSecondLine);  // push second line
array_unshift($file, $first_line);      // Save back the first line

$fp = fopen('yourfile.php', 'w');       // Reopen the file
fwrite($fp, implode("n", $file));
fclose($fp);