problems when extending a PHP Class

I suppose you want to call the parent function so try somenthing like that

 function Total($table) {
      $query = parent::Execute("SELECT * FROM $table");
      $total = parent::NumRows($query);
      return $total;
    }

And if it doesnt't work try with this instead of parent

PHP parent codumentation

Well, yes, $DB is not defined/in scope within the Total function.
If $DB is a property of the class, you'll need to access it with $this->DB.

Update after update:

You're instantiating the DBManager into the global variable $DB. This variable is only in scope in the global scope. It is not automatically available in any other scope, like inside functions. Whether or not User inherits from DBManager is completely irrelevant, the instantiated variable $DB is not in scope inside any function.

There's a lazy way to access variables in the global scope, but I'm not going to mention that here. The proper way would be to instantiate $DB as a class member of User or pass it upon instantiating User:

class User {

    protected $DB = null;

    public function __construct($DB) {
        $this->DB = $DB;
    }

    public function Total() {
        $this->DB->…
    }
}

$DB = new DBManager();
$user = new User($DB);

What is that $DB? You never set that to anything, so even if you include those files, you cannot reach that variable

Depending on your db.class.php, you probably need to instantiate some sort of $DB like this

$DB = new db();

(either in the function itself, or in a constructor for the user class using $this->DB)

try something like this:

Class User extends DBManager {
    private $DB;

    function User() { // should be __construct() really
        $this->DB = new DBManager();
        $this->DB->Connect();
    }

    function Total($table) {
      $query = $this->DB->Execute("SELECT * FROM $table");
      $total = $this->DB->NumRows($query);
      return $total;
    }


}

$User = new User();

Looks like you're confused about how inheritance works. The sub-class inherits the public and protected members and methods of the parent class. Therefore, in the example you've given, you should probably be using $this->Execute("SELECT * FROM $table") and $this->NumRows($query). Although the Execute and NumRows functions aren't defined in the User class, they are in the DBManager class, so you access them as if they were defined in User.

The solution for my issue is:

$query = DBManager::Execute("SELECT * FROM $table");
$total = DBManager::NumRows($query);