I wanted to know the difference between the presence and absence of the first condition of the code below

Question

I have a code that I picked up somewhere, some time ago, that I use to make connections with my database and it always worked well. I tried, then, to remove a piece of code that I didn’t understand and, to my surprise, it kept working.. or so I thought. Throughout the project, I was obliged to use the lastInsertId() function, from PHP, but always returned 0, regardless of the command I tried to execute before in the database. I spent a lot of time understanding that what was missing for the code to work as I wanted, returning the value of the last id inserted in the table of that database, was the line of code that I had removed, which is a condition.

Below, the code that works:

class MySql{
private static $pdo;

protected function MySql(){
    if (self::$pdo == null) {
        try {
            self::$pdo = new PDO('mysql:host='.BD['host'].';dbname='.BD['name'],BD['user'],BD['password'],array(PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES utf8"));
            self::$pdo->setAttribute(PDO::ATTR_ERRMODE,PDO::ERRMODE_EXCEPTION);
            
        } catch (Exception $e) {
            echo '<h2>Erro ao conectar</h2>';
        }
    }
    return self::$pdo;
}

}

Now, what doesn’t work:

class MySql{
private static $pdo;

protected function MySql(){
        try {
            self::$pdo = new PDO('mysql:host='.BD['host'].';dbname='.BD['name'],BD['user'],BD['password'],array(PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES utf8"));
            self::$pdo->setAttribute(PDO::ATTR_ERRMODE,PDO::ERRMODE_EXCEPTION);
            
        } catch (Exception $e) {
            echo '<h2>Erro ao conectar</h2>';
        }
    return self::$pdo;
}

}

Now, what I used to try to get the last id:

$sql = $this->MySql()->prepare("$query");
$sql->execute($parametros);
$ultimo_id = $this->MySql()->lastInsertId();
        

Where $query is an insertion query and $parameters is an array containing the values - obviously, the table must have a primary key, which I always call 'id'. The first code works well, while the second always returns 0. The only difference between them is the if condition. I wanted to understand what the difference is between the two and why this difference happens. I would appreciate it if someone could explain to me.