I am using $this in a class, but it keeps giving me Fatal error: Using $this when not in object context

Question

I created a a cart class which contains songs to be bought. The cartSong class works fine, but when I use the cart class, an error about $this always comes up. I want the variable $songList (array) to add a song object to the cart every time addToCart is called, and $trackno to iterate. The line where the error is located is specified in the code:

<?php
$indexpath = "index.php";
$cartpath = "data/cart.xml";

class cartSong{
    private $albumid = null;
    private $trackno = null;

    function cartSong($albumid, $trackno){
        $this->albumid = $albumid;
        $this->trackno = $trackno;
    }

    function setSong($albumid, $trackno){
        $this->albumid = $albumid;
        $this->trackno = $trackno;
    }

    function getTrackNo(){
        return $this->trackno;
    }

    function getAlbumID(){
        return $this->albumid;
    }
}

class cart{
    public $songList;
    public $songCount;

    function cart(){
        $this->songList = array();
        $this->songCount = 0;
    }

    function addToCart($albumid, $trackno){
        $checker=0; 

        for($i=0;$i<$this->songCount;$i++){ // THIS LINE GIVES AN ERROR ($this->songCount)
            if( ($this->songList[$i]->getAlbumID()==$albumid) && ($this->songList[$i]->getTrackNo()==$trackno) )
                $checker=1;
        }

        if($checker==0){
            $song = new CartSong($albumid, $trackno);
            $this->songList[]=$song;
            $this->songCount++;
        }
        else
            echo "Song already exists in cart.";
        echo $this->songList[0]->getAlbumID();
        echo $this->songList[0]->getTrackNo();
    }

    function removeFromCart($albumid, $trackno){
        $checker=0;

        for($i=0;$i<count($songList);$i++){
            if( ($songList[$i].getAlbumId()==$albumid) && ($songList[$i].getTrackNo()==$trackno) )
                $checker=1;
        }

        if($checker==1){
            array_splice($songList,$i);
        }
        else
            echo "Song does not exist in cart.";
    }

    function emptyCart(){
        $songList = (array) null;
    }
}

?>

There is only one error when I run this:

Fatal error: Using $this when not in object context in C:\wamp\www\musiquebasse\data\cartfunctions.php on line 40.

Here is where I called the code, this is addtocart.php:

<?php
$indexpath = "index.php";
require_once "data/SessionControl.php";
require_once "data/cartfunctions.php";

    $album = $_GET["albumid"];
    $track = $_GET["trackno"];
    $action = $_GET["action"];
    $cart = new cart();

    // insert checker here (if the same song is added to cart

    switch($action) {   //decide what to do 
        case "add":
            $cart::addToCart($album, $track);
        break;

        case "remove":
            $cart::removeFromCart($album, $track);          
        break;

        case "empty":
            $cart::emptyCart();
        break;

    }

?>

Answer 1

You are calling addToCart as a static method using the :: operator in your code:

$cart::addToCart($album, $track);

Instead you should reference the function against the instantiated object, using the -> operator:

$cart->addToCart($album, $track);

You have the same problem with the remove and empty calls as well.

Edit: I see that you already fixed in the comments - I'll just leave this here I guess.