$_db ='SMS';
$_user ='root';
$_pass ='';
$_host ='localhost';

$command = 'C:\wamp\bin\mysql\mysql5.6.12\bin\mysqldump.exe --opt -u '.$_user.' -p'.$_pass.' '.$_db.' > c:\\smsexport.sql';

exec($command);

echo "<br />".$command;