$user = (!empty($_POST['user'])) ? $_POST['user'] : '';
$pass = (!empty($_POST['pass'])) ? $_POST['pass'] : '';

$res = mysql_query("SELECT * from users where user='{$user}' AND pass='{$pass}'");

$res = mysql_query("SELECT * from users where user='foo' -- ' AND pass=''");

$res = mysql_query("SELECT * from users where user='foo' OR (DROP TABLE users) -- ' AND pass=''");

$res = mysql_query("SELECT * from users where user='foo'; DROP TABLE users -- ' AND pass=''");

For both username and password, I enter:

' OR 1=1 AND '}' '=

$user = (!empty($_POST['user'])) ? $_POST['user'] : '';
$pass = (!empty($_POST['pass'])) ? $_POST['pass'] : '';

$user = mysql_real_escape_string($user);
$pass = msyql_real_escape_string($pass);

$res = mysql_query("SELECT * from users where user='{$user}' AND pass='{$pass}'");