Question

The parent class has a constructor like this:

function CD_Log( $id = false ) {
    $this->__construct( $id );
}

function __construct( $id = false ) {
    global $cd;

    if ( !empty( $id ) ) {
        $this->id = $id;
        $this->populate();
    }
}

In the extended Class, I want to save some of parent’s var and child’s var to a different place, also won’t use the populate() function. Other than that, the child is constructed the same way as parent. Should I write a contructor for the child?

Answer 1

If you are following OOP as you are now, you should remove the global call for $cd, set this as a member var of either the parent or child.

Constructors, as I have seen them commonly used are normally called when you instantiate the object, not through another method of the same class, like the method CD_Log is doing.

For example:

$myCdObject = new MyCdClass(); // Automatically calls constructor
$myCdObject->Cd_Log($string);

With the above example you are ensure the CD_Log method is only dealing with log logic. Unless you make CD_Log a static function, then you can instantiate the class within:

public static function CD_Log($id)
{
    $myCdObject = MyCdClass(); // Automatically calls constructor

    // Continue with `log` logic
}

To answer your question you probably want a constructor in the child class, which will call the parent constructor explicitly:

class parent
{
    public function __construct($id)
    {
        $this->id = $id;
    }
}

class child extends parent
{
    public function __construct($id)
    {
        // Going to take advantage of shared logic
        parent::__construct($id);

        // Now lets do some child class specific stuff
    }
}