help please T.T

how to target player like this

/set <playername> life 20

like that?

i only ask the <playername> i dont know what is that called but i want to target player to give or set what i want! what code for that?

 

PHP:

$playerName = $args[0];
$player = $this->getServer()->getPlayer($playerName);
if ($player !== null) {
  $player->setHealth((int) $args[2]);
} else {
  $sender->sendMessage("Player not found");
}