What is the difference between these 2 PHP codes?

Question

I am studying PHP and came across the following question: what would be the difference between the following codes:

<?php
class ClasseTeste
{
    protected $db_host = 'localhost';
    protected $db_user = 'root';
    protected $db_pass = 'root';
    protected $db_name = 'bd_nome';

    public function hello()
    {
        echo $this->db_host;
    }
} 

And this:

<?php
class ClasseTeste
{
    protected function __construct()
    {
        $this->db_host = 'localhost';
        $this->db_user = 'root';
        $this->db_pass = 'root';
        $this->db_name = 'bd_nome';

        /**
         * Gostaria de saber se eu poderia tambem definir o tipo dos campos dentro da funcao __construct(). Dessa maneira:
         *
         * private $this->db_host = 'localhost';
         * private $this->db_user = 'root';
         * private $this->db_pass = 'root';
         * private $this->db_name = 'db_name';
         */
    }

    public function hello()
    {
        echo $this->db_host;
    }
}

Answer 1

The first code explicitly defines the attributes as members of the class and their visibility is also guaranteed (protected) all of them are accessible within the class and their descendants as expected.

The second defines the constructor as protected, that is, it cannot be called directly, in which case it would be necessary a static and public method that called the constructor and at the end returned the desired object. This mechanics reminds us a little bit of the Singleton standard

The second code in the constructor are defined class members in a dynamic way that at first glance has the result of the first code but is not so because the attributes created in this way have their visibility as public, that is, they are accessible by anyone and this violates the encapsulation.

What is possible to do in the second code and not first.

//nesse exemplo mudei o construtor para public para teste
$obj = new ClasseTeste();
echo $obj->db_pass .' - '. $obj->db_user;

Answer 2

The only functional difference is to define whether it is public or protected. The rest is just the same organizational issue. There are people who just prefer to start the variable outside the function and set the values in the function __construct. But it makes no difference.

I wonder if I could also define the type of fields within the __Construct() function. That way:

    private $this->db_host = 'localhost';
    private $this->db_user = 'root';

No, in that case it would have to be out of function.

*Obs.: The function __construct must be public and it is not mandatory to declare before, can be created within it.

Answer 3

The first code works as follows:

You got a class ClasseTeste with 4 protected attributes, attributes referring to access to a database and public function that when called prints the attribute db_host.

The second code works as follows: When the class is being instantiated it assigns the strings to each attribute and public function that when called prints the attribute db_host. Works with caveats, it is not possible to define the stereotype (Protect, private, public).

<?php
class ClasseTeste
{
 protected $db_host;
    protected $db_user;
    protected $db_pass;
    protected $db_name;

    protected function __construct()
    {
        $this->db_host = 'localhost';
        $this->db_user = 'root';
        $this->db_pass = 'root';
        $this->db_name = 'bd_nome';
    }

    public function hello()
    {
        echo $this->db_host;
    }
}
$objeto = new ClasseTeste();
$objeto->hello();
?>