Calling a child object in a parent function

I would like to know If I can call a child's object in a parent function. Like this:

   class Parent {

          public function A() {
             << I want to call the object here >>
             // Some code 
          }
   }

   Class Child extends Parent {

          public function B() {
            // Some code
          }
   }

   $Child = new Child();
   $Child -> B();

The two classes are in different files. My Child class is making a connection with my database with function B(). In my Parent's class, A(), I am trying to insert data I receive from filling a form, but I need a connection to the database and I do not know how I can call that object. Note: My code is working when I have both functions in the same Class.

I did not find the solution, so I will try and post my real code:

 class db_connect extends Model
 {
    private $dbname = "...";
    private $dbuser = "...";
    private $dbpass = "...";
    private $dbhost = "...";
    public $dbc;

    public function Connect()
    {
        $this->dbc = mysqli_connect($this->dbhost, $this->dbuser, $this->dbpass, $this->dbname);

    if($this->dbc === false){
        die("ERROR: Could not connect. " . mysqli_connect_error());    
    }}
}

So this was the Class Child from Above and Connect() was B().

Now the parent

class Model 
{
      public $query;
      public $result;

      public function proccess_data($marca,$model,$pret,$descriere){
         << I am trying to make a connection here from config.php using the function Connect() >>
         $this->query = "INSERT INTO autoturisme (autoturism_id, marca, model, pret, descriere) " .
     "VALUES (NULL, '$marca', '$model', '$pret', '$descriere')";
         $this->result = mysqli_query(<<Also here I would need the connection>>, $this->query) 
    or die(mysqli_error(<<And here>>));
         if($this->result == 1){
         echo "<br/>Your data were processed";
    } else {
         echo "<br/>We are sorry but an error occurred";
    }
        $this->close_db();

}

In the mysqli_query I need a parameter as a mysqli, which is the connection to my database. The parameter is in the Child's Class, $dbc, and it is called in the function Connect() with: $this->dbc . Same goes for mysqli_error. Hope this makes things clearer :).