$player->knockback

I know I can use $player->knockBack($damager,$damage,$x,$z,$base);

$damager can be $player
$damage will be 0
$x and $z: are those variables yaw&pitch + If yes, will just setting 0,0 works (knockback to the direction you're looking)
$base is number (I use 0)

Actually, the first two parameters are there as an accident. Due to backwards compatibility we don't remove it. They are actually unused.

And what about $x,$z, so it's pitch&yaw?

no it is x and z

motion X and motion Z

Is able to define it from $ev->getTo()?

you can make knockback which you want

Learn trigonometry and read the PocketMine code about it.

I did. However, I don't know how :/

https://github.com/PocketMine/PocketMine-MP/blob/master/src/pocketmine/entity/Living.php#L136-153