Question

I’m trying to develop a system using MySQL and PHP. I’m using mysqli* function using OOP. The problems I’m having is the next one:

I have a class for connect to the database:

<?php
  class Conexion{
   $this->linkDb = new mysqli("127.0.0.1", "user", "pass");
   if($this->linkDb){
     $this->linkDb->select_db("test");
     return TRUE;
   }
  else{
     return FALSE;
  }
 } // End of class Conexion

?>

I have several classes to retrieve and store data in my database, for example:

<?php
class Product{

  public function listProducts()
  {
    $query = "SELECT * FROM products";
    $rsProducts = $linkDb->query($query);
    if($rsProducts){
       // ....
    }
    else{
      // ....
    }
  }
}
?>

And I use this classes in the next file (main):

<?php
  function __autoload($className)
  {
    require_once("./class/$className.php");
  }

  $cnx = new Conexion();
  $prd = new Products();
  if($prd->listProducts()){
    echo "OK";
  }
  else{
    echo "Wrong...";
  }
?>

When I try to run this file, I get the next error:
Fatal error: Call to a member function query() on a non-object in /home/emco/training/phpmysqli/class/Products.php on line 8

I know the error is because of the scope of $linkDb in the method listProducts(). My question is, How can I do to make the link of the conexion available in all the functions of my classes? Do I have to make a conexion link in each of the functions of my classes? What’s the correct way to do this?

Answer 1

You can either pass the $cnx object to each method call of Products, or establish it as a property of Products. (A third method would be to declare global $linkDb inside methods, but it is clearer to use a dependency injection.)

Example 1: Inject the connection into the constructor and keep it as a property to Products

class Product{
  // Database as property
  public $linkDb;

  // Pass it into the constructor
  public function __construct($cnx) {
    $this->linkDb = $cnx;
    // ...other stuff...
  }

  public function listProducts()
  {
    $query = "SELECT * FROM products";
    // Access it with $this->linkDb
    $rsProducts = $this->linkDb->query($query);
    if($rsProducts){
       // ....
    }
    else{
      // ....
    }
  }
}

// Use it as:
$cnx = new Conexion();
$prd = new Products($cnx);

Example 2: Pass $cnx to each method call of Products where it is needed

// Require one parameter $linkDb
public function listProducts($linkDb)
{
  $query = "SELECT * FROM products";
  $rsProducts = $linkDb->query($query);
  if($rsProducts){
   // ....
  }
  else{
    // ....
  }
}

// Call as
$cnx = new Conexion();
$prd = new Products();

// Pass $cnx as a parameter
if($prd->listProducts($cnx)){
  echo "OK";
}