JSONized.php

trait JSONized {

    public function toJson()
    {
        $properties = get_object_vars($this);
        return json_encode($properties);
    }

}

test.php

include_once 'JSONized.php';
include_once 'User.php';

$hede = new User('h', 'm', 'hede@hede.com');
echo $hede->toJson();

User.php

class User
{
    use JSONized;

    public $username;
    public $password;
    public $email;

    public function __construct($username, $password, $email)
    {
        $this->username = $username;
        $this->password = md5($password);
        $this->email = $email;
    }
}