php – How can i connect to database with mysqli without sql injection-ThrowExceptions

i’m learning about telegram bots from a tutorial and the tutorial uses mysqli for connecting to database.
but some parts of code are confusing me and i cannot understand them
in index.php file :

$db = Db::getInstance();
     $jok = $db->query("SELECT * FROM jok ORDER BY RAND() LIMIT 1");

we call getInstance() function and store its value in $db.but in db.php file we see that $db is private property

db.php :

class Db {

  private $connection;
  private static $db;



  public static function getInstance($option = null){
    if (self::$db == null){
      self::$db = new Db($option);
    }

    return self::$db;
  }

also i cannot understand this part of code too :

public static function getInstance($option = null){
    if (self::$db == null){
      self::$db = new Db($option);
    }