$X = $_POST['X'];
$Y = rand(1, 3);
$A = $X-$Y+3;
$R = fmod($A, 3);

if ($R == 0) {
    echo 'draw!';
} elseif ($R == 1) {
    echo 'you won!';
} elseif ($R == 2) {
    echo 'you lost!';
} else {
    echo 'an error occurred!';
}

Table: Users

UserID    Username    (I don't include email, password and salt here...)
1         Espen
2         Test

Table: Games

GameID    Participant1 Participant2 Winner
1         1            2            1
1         1            2            2

Table: Moves

MoveID    GameID       UserID       Move
1         1            1            2
2         1            2            1
3         2            1            3
4         2            2            1