[php] insert password into database in md5 format?

You can use MD5() in mysql or md5() in php. To use salt add it to password before running md5, f.e.:

$salt ='my_string';
$hash = md5($salt . $password);

It's better to use different salt for every password. For this you have to save your salt in db (and also hash). While authentication user will send his login and pass. You will find his hash and salt in db and find out:

if ($hash == md5($salt . $_POST['password'])) {}