ArrayAccess Walkthrough with Questions

Question

I have some questions about the implementation of implementing ArrayAccess in PHP.

Here is the sample code:

class obj implements arrayaccess {
    private $container = array();
    public function __construct() {
        $this->container = array(
            "one"   => 1,
            "two"   => 2,
            "three" => 3,
        );
    }
    public function offsetSet($offset, $value) {
        if (is_null($offset)) {
            $this->container[] = $value;
        } else {
            $this->container[$offset] = $value;
        }
    }
    public function offsetExists($offset) {
        return isset($this->container[$offset]);
    }
    public function offsetUnset($offset) {
        unset($this->container[$offset]);
    }
    public function offsetGet($offset) {
        return isset($this->container[$offset]) ? $this->container[$offset] : null;
    }
}

Questions:

1. I am not asking why we do have to implement ArrayAccess since I am assuming it is special interface that PHP Engine recognizes and calls the implemented inherited functions automatically?
2. Why are we declaring the implemented function public? Since I assume they are special functions called automatically. Shouldn't they be private since when calling saying $obj["two"] the functions are not be called from outside.
3. Is there a special reason to assign the filled-array in __constructor function? This is the constructor function I know but in this case what kind of help it is being of.
4. What's the difference between ArrayAccess and ArrayObject? I am thinking the class I implemented by inheriting the ArrayAccess doesn't support iteration?
5. How could we implement object-indexing without implementing ArrayAccess?

Thanks...

Answer 1

1. Correct
2. Because the interface defines them as public, therefore you have to also
3. You don't have to write the constructor that way if you don't want to*
4. ArrayAccess is an interface, ArrayObject is a class (which itself implements ArrayAccess)
5. No other way that I'm aware of

* Your constructor could look like this

public function __construct( array $data ) {
    $this->container = $data;
}