php - Outputting form data via classes

I'm trying to figure out how to output data from a form using classes. I've validated my form data in a validation PHP file and i'm using another file called database to output the text from my form to a file. I've tried using the function 'init' with multiple arguments: resulted in a blank file. Tried to hardcode the data: also blank.

class Database implements JsonSerializable
{
    public $name;
    public $age;
    public $email;
    public $phone;
    public $file = "database.txt";
    private $filehandler;
    public function __construct($name, $age, $email, $phone)
    {
        $this -> name = $name;
        $this -> age = $age;
        $this -> email = $email;
        $this -> phone = $phone;
    }
    public function init(){
        $this -> filehandler = fopen($this -> file, "w");
        fwrite($this -> file -> $this->filehandler, "Help");
        fclose($this -> filehandler);
    }
    public function jsonSerialize()
    {
       return ['name' => $this -> name,
                'age' => $this -> age,
                'email' => $this -> email,
                'phone' => $this -> phone
       ];
    }
}
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Object creation:

$database = new Database($_POST['name'], $_POST['age'], $_POST['email'], $_POST['phone']);

$database->jsonSerialize();
$database->init();
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