Decode Json data into PHP Variables

Im sending my data to PHP file like this using angularJS ajax

{ username: "someusername", password: "somepassword" }

previously i used something like this to send data

var vars = "username="+fn+"&password="+ln;

but since angular sending data like JSON. i want to decode into PHP variables.

here my PHP code

if(isset($_POST['username']) && isset($_POST['password'])){
    $username = $_POST['username'];
    $password = $_POST['password'];
}

$sql = "SELECT * FROM member WHERE username = '$username' AND password = '$password'";

what i tried to do is something like this

$array = json_decode(file_get_contents('php://input'))
    $username = $array->username;
    $password = $array->password;


$sql = "SELECT * FROM member WHERE username = '$username' AND password = '$password'";

but not sure how to do it. also no idea about what happens in this section 'file_get_contents('php://input')' im sending data from local computer to internet server PHP file (Cross Domain)