[php] MySQL Check if username and password matches in Database

I have a form which has a textbox with the name attribute username and another one with the name attribute password. I also have a database with columns called user and pass. When my users signed up it added the username to the user column and password to the pass column.

How would I make a MySQL query to check if the form submitted the right username and password and then if it did have a branch to let me input the code for if it succeeded?

I really need some code, this bit isn't going well I know it should be something like SELECT * FROM table WHERE username == $username AND... but then I'm stuck because I have an MD5 password in the database and that first bit is probably wrong. Please help. :)

Thanks

//set vars
$user = $_POST['user'];
$pass = md5($_POST['pass']);

if ($user&&$pass) 
{
//connect to db
$connect = mysql_connect("$server","$username","$password") or die("not connecting");
mysql_select_db("users") or die("no db :'(");
$query = mysql_query("SELECT * FROM $tablename WHERE username='$user'");

$numrows = mysql_num_rows($query);


if ($numrows!=0)
{
//while loop
  while ($row = mysql_fetch_assoc($query))
  {
    $dbusername = $row['username'];
    $dbpassword = $row['password'];
  }
  else
      die("incorrect username/password!");
}
else
  echo "user does not exist!";
} 
else
    die("please enter a username and password!");

Instead of selecting all the columns in count count(*) you can limit count for one column count(UserName).

You can limit the whole search to one row by using Limit 0,1

SELECT COUNT(UserName)
  FROM TableName
 WHERE UserName = 'User' AND
       Password = 'Pass'
 LIMIT 0, 1