PHP - 两个类，正确地相互交谈

Question

I have a small problem. I've tried searching, but I can't get the search terms quite right and was hoping someone could help.

I have an include on every page in my system that works something like this:

<?PHP
require_once("class.system.php");
require_once("class.mysql.php");

$oMySQL = new MySQL();
$oSystem        = new SystemClass();

... ?>

But I have a problem. As you may guess - the MySQL class is a bunch of functions that I use to make MySQL calls easier. This isn't the only example of where I want to use it but it's a good example.

I have functions in the system class I want to be able to reference the MySQL class (And vice versa...).

As an example, I have a function in the system class that will populate a session variable with data from MySQL. The only way I can think of doing this (Which I know is wrong...) is:

class SystemClass {

    function PopulateSession(){
         global $oMySQL;
         if($oMySQL->Select('abc')){
             $_SESSION['def']= blahblahblah;
             return true;
         } else {
             return false;
         }
    }
}

It works, but it means every function I want to use it, I have to use global, which I'm sure is very bad practice. Could someone advise??

Thanks

Answer 1

What you encountered is called composition. A good solution would be to use a dependency injection framework. An easy solution is to roll with constructor parameters.

public class A { 
  private $b;
  public function __construct($b) {
    $this->b = $b;
  }
}

$b = new B;
$a = new A($b); 

Or, as a more flexible solution, when you have mutual dependencies:

public class A {
  private $b;
  public function setB($b) {
    $this->b = $b;
  }
}

public class B {
  private $a;
  public function setA($a) {
    $this->a = $a;
  }
}

$a = new A;
$b = new B;
$a->setB($b);
$b->setA($a);

But the downside is that as the number of dependencies grows, it's hard to manage and remember to set all the dependencies. This is exactly the reason why Dependency Injection frameworks are popular.