Chain method php

Sekarang saya lagi membuat project "QueryBuilder", fitur yang sama kek di laravel.

Ini file utamanya:

class Query extends DB {

    private static $tb;
    private $query_stmt;
    private $key;
    private $trigger_code;

    public static function table(string $arg) {
        self::$tb = $arg;
        return new static;
    }

    public function select(array $column) {
        $column_stmt = "";
        if(is_array($column)) {
            foreach($column as $q) {
                if($q != $column[array_key_last($column)]) {
                    $column_stmt .= $q.",";
                } else {
                    $column_stmt .= $q;
                }
            }
            $this->query_stmt = "SELECT $column_stmt FROM ".self::$tb;
            return $this;
        }
    }

    public function where(array $arg) {
        if(is_array($arg)) {
            $this->key = $arg[array_key_first($arg)];
            $this->query_stmt .= " WHERE ".array_key_first($arg). " = ?";
            $this->trigger_code = 1;
            return $this->exec();
        }
    }

    public function like(array $arg) {
        if(is_array($arg)) {
            $this->key = $arg[array_key_first($arg)];
            $this->query_stmt .= " WHERE ".array_key_first($arg). " LIKE ?";
            $this->trigger_code = 2;
            return $this->exec();
        }
    }

    public function exec() {
        $stmt = $this->koneksi->prepare($this->query_stmt);
        if($this->trigger_code == 1) {
            $stmt->execute([$this->key]);
            return $stmt->fetch(PDO::FETCH_ASSOC);
        } else if($this->trigger_code == 2) {
            $stmt->execute(["%".$this->key."%"]);
            return $stmt->fetchall(PDO::FETCH_ASSOC);
        }
    }

}

Ini cara penggunaan, menggunakan select sekaligus where

$x = Query::table("dataapi")
            ->select(["id","username"])
            ->where(["username" => "ahmad"]);
print_r($x);

// Hasil nya berhasil
// Outputnya seperti ini
Array ( [id] => 29 [username] => Ahmad )

tapi, ketika saya menggunakan fitur select doang

$x = Query::table("dataapi")
            ->select(["id","username"]);

print_r($x);

Outputnya kaya gini:

Query Object ( [koneksi:protected] => PDO Object ( ) [query_stmt:Query:private] => SELECT id,username FROM dataapi [key:Query:private] => [trigger_code:Query:private] => )

Kenapa seperti itu ya?

ekspektasi saya sih, harusnya me-return hasil keseluruhan.

Maaf kepanjangan pertanyaannya, soalnya saya bingung mau nanya dimana lagi