Question

Hi guys I am having problem with my code. with __construct() I am getting Fatal error: Call to a member function prepare() on a non-object but without it my code is working.

class Animals{

public $db_fields;

 public function __construct(){
  $this->db_fields = $this->get_fields();

  foreach($this->db_fields as $field){
  $this->$field = "";
 }


 public function get_fields(){
  global $dbh;

  $q = $dbh->prepare("DESCRIBE animals");
  $q->execute();
  $db_fields = $q->fetchAll(PDO::FETCH_COLUMN);

  return $db_fields;
 }
}
$f = new Animals();

/*** mysql hostname ***/
$hostname = 'localhost';
/*** mysql username ***/
$username = 'root';
/*** mysql password ***/
$password = '';
/*** mysql database***/
$dbname = 'animals';

try {
$dbh = new PDO("mysql:host=$hostname;dbname=$dbname", $username, $password);
/*** echo a message saying we have connected ***/
echo 'Connected to database <br />';

$dbh = null;
}
catch(PDOException $e)
{
echo $e->getMessage();
}

I just want to make my fields(animal_id,animal_type,animal_name) work as same as

public $animal_id;
public $animal_type;
public $animal_name;

Answer 1

You have to instantiate the Animals object after you create $dbh, otherwise it’s not defined in the get_fields() function.

$dbh = new PDO("mysql:host=$hostname;dbname=$dbname", $username, $password);
$f = new Animals();

That being said, a better design is to use dependency injection, and send the $dbh object to the class, like this:

$dbh = new PDO("mysql:host=$hostname;dbname=$dbname", $username, $password);
$f = new Animals( $dbh);

So, your updated class would look like:

class Animals{

    private $dbh;

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

    public function get_fields(){
        $q = $this->dbh->prepare("DESCRIBE animals");
    }
}

The benefit? You get rid of the global variable. Typically, requiring a global variable is a bad design, and you want to avoid it. You can see why just based on the problem you’re having – It requires a global state to be set in order to function.