OO + PHP + Array iSSUE

The problem is you're always assigning the values to $this, then store it in an array. Objects are always stored as reference, so you're just storing a reference to the $this object over and over in the $productArray. When you finally print the content of the array, you only have the same object in it several times which contains the last data you have assigned to it.

That var_dump($productArray[0]) should work seems suspect, maybe you're calling it within the loop before the values got overwritten?

I can't fully explain the behaviour, but the cause is this line:

$productArray[$count] = $this;

You are re-using the $this special variable to just store some static data. Replace it with some other variable name in the while loop (like $data) and you should be fine:

    while($row = mysql_fetch_row($result)){
        $properties = array
        (
            'id', 'category_id', 'title', 'short_description', 'long_description'
            , 'tag', 'price', 'weight', 'stock'
        );
        $data = (object) array_combine($properties, $row);   
        $productArray[$count] = $data;
        $count++;
    }

If you take $this, each time that specific object instance changes, the data inside the array will change. You get a sort of unpredictable behavior (as you notice it already).

The usage of $this is a special variable that always passes the Object by reference. The continued usage of this will change the current object's state throughout the loop and assign a reference to the current object into the array which would result in an end result of an array with X number of references to the same object.

The difference with the $productArray[$var] is just coincidence. If you run it with

echo '<pre>';
print_r($productArray);
echo '/n';
print_r($productArray[0]);
echo '/n';
print_r($productArray[1]);
echo '</pre>';

You should notice that they are all the same object output.

If you created a new object instead it would function as you expect it.

public function getAllProducts($limit){

    $sql = "SELECT * FROM products
            ORDER BY RAND()
            LIMIT $limit";

    $result = mysql_query($sql) or die(mysql_error());

    $productArray = array();
    $count = 0;

    while($row = mysql_fetch_row($result)){
        $that = new Product; // Whatever your product object would be or stdClass.
        $that->id = $row[0];
        $that->category_id = $row[1];
        $that->title = $row[2];
        $that->short_description = $row[3];
        $that->long_description = $row[4];
        $that->tag = $row[5];
        $that->price = $row[6];
        $that->weight = $row[7];
        $that->stock = $row[8];   
        $productArray[$count] = $that;
        $count++;
    }

    echo'<pre>';
    print_r($productArray);
    echo'</pre>';

    return $productArray;

}

I would approach this by adding a function to return a newly instantiated object instead of assigning public variables. This can allow much easier maintenance of the code if you decide to have special methods upon creation of the object.

public static function getProductFromRow(Array $row) {
    $that = new Product; // Whatever your product object would be.
    $that->id = $row[0];
    $that->category_id = $row[1];
    $that->title = $row[2];
    $that->short_description = $row[3];
    $that->long_description = $row[4];
    $that->tag = $row[5];
    $that->price = $row[6];
    $that->weight = $row[7];
    $that->stock = $row[8];

    return $that;
}

public function getAllProducts($limit) {

    $sql = "SELECT * FROM products
            ORDER BY RAND()
            LIMIT $limit";

    $result = mysql_query($sql) or die(mysql_error());

    $productArray = array();
    $count = 0;

    while($row = mysql_fetch_row($result)){
        $productArray[$count] = Product::getProductFromRow($row);
        $count++;
    }

    return $productArray;

}