PHP, SPL, ArrayAccess Interface

Question

I am trying to understand the idea behind ArrayAccess Interface,

I dont understand what each method is about, If those methods(functions) are "built in" functions and ArrayAccess Interface(also "built in") is only "make sure" i am going to implement those "built in" methods(functions)

I am trying to understand what does each of thoes functions is doing with our code "Behind the scenes".

function offsetSet($offset, $value);
function offsetGet($offset);
function offsetUnset($offset);
function offsetExists($offset);

If i understand ArrayAccess is a Built In interface that Containing seals to implement, when we implement them we only implement references to thoes built in functions, I will be happy if some one can please help me get this right.

Answer 1

If you implement that interface, then the object acts like an array. e.g., if $foo is an instance of a class that implements ArrayAccess:

$foo['bar'] = 42 calls offsetSet('bar', 42).

echo $foo['bar'] calls offsetGet('bar').

unset($foo['bar']) calls offsetUnset('bar').

isset($foo['bar']) calls offsetExists('bar').

You never explicitly call the functions offset* yourself. It happens implicitly when you access the object as an array.

Answer 2

While comparing ArrayAccess to SimpleXMLElement (an internal class not implementing it), I was curious, too. The interface is well documented in the manual already, so I wanted to highlight some differences in specific with offset types.

But first of all a boilerplate example implementation of a class implementing ArrayAccess giving output when accessed:

/**
 * ArrayAccess Example
 */
class ExampleArrayLikeAccess implements ArrayAccess
{

    /**
     * Whether a offset exists
     *
     * @link http://php.net/manual/en/arrayaccess.offsetexists.php
     * @param mixed $offset - An offset to check for.
     * @return boolean true on success or false on failure.
     *
     * The return value will be casted to boolean if non-boolean was returned.
     */
    public function offsetExists($offset) {
        echo "  - offsetExists(", $this->varString($offset),")\n";
    }

    /**
     * Offset to retrieve
     *
     * @link http://php.net/manual/en/arrayaccess.offsetget.php
     * @param mixed $offset The offset to retrieve.
     * @return mixed Can return all value types.
     */
    public function offsetGet($offset) {
        echo "  - offsetGet(", $this->varString($offset),")\n";
    }

    /**
     * Offset to set
     *
     * @link http://php.net/manual/en/arrayaccess.offsetset.php
     * @param mixed $offset The offset to assign the value to.
     * @param mixed $value The value to set.
     * @return void
     */
    public function offsetSet($offset, $value) {
        echo "  - offsetSet(", $this->varString($offset), ", ", $this->varString($value), ")\n";
    }

    /**
     * Offset to unset
     * @link http://php.net/manual/en/arrayaccess.offsetunset.php
     * @param mixed $offset  The offset to unset.
     * @return void
     */
    public function offsetUnset($offset) {
        echo "  - offsetUnset(", $this->varString($offset),")\n";
    }

    /**
     * helper to give a variable dump in form of a string
     */
    private function varString($var) {
        ob_start();
        var_dump($var);
        return trim(strtr(ob_get_clean(), ["\n" => '', "\r" => '']), ' {}');
    }

}

Running some usage-examples with it. I have left notes in form of comments. It should be pretty self-explaining:

$like = new ExampleArrayLikeAccess();


/* offsetExists */

// indexes/keys that behave similar to PHP arrays:

isset($like[1]);    # integer stay integer
# offsetExists(int(1))

isset($like['1']);  # string like an integer - converted to integer
# offsetExists(int(1))

isset($like['01']); # string unlike an integer - stays string
# offsetExists(string(2) "01")

isset($like[TRUE]); # booleans are converted to integer
# offsetExists(bool(true))

// indexes/keys that differ to PHP arrays:

isset($like[1.1]);     # a float stays a float (double)
# offsetExists(double(1.1))

isset($like[NULL]);    # NULL stays NULL
# offsetExists(NULL)

isset($like[array()]); # array stays array
# offsetExists(array(0))

isset($like[$like]);   # object stays object
# offsetExists(class SxeLikeAccess#2 (0))


/* offsetGet */

// indexes/keys behave the same as with offsetExists:
$like[1];    # offsetGet(int(1))
$like['1'];  # offsetGet(int(1))
$like['01']; # offsetGet(string(2) "01")
// ...


/* offsetSet */

$like[1] = 'value';    # index/key behaves the same as with offsetExists
# offsetSet(int(1), string(5) "value")

$like[] = 'value';     # index/key is NULL
# offsetSet(NULL, string(5) "value")

$like[NULL] = 'value'; # index/key is NULL
# offsetSet(NULL, string(5) "value")


/* offsetUnset */
unset($like[1]);       # index/key behaves the same as with offsetExists
unset($like[NULL]);    # same for NULL

Key differences to standard PHP arrays are that you can use not only integer and string as offsets.