<?php
$arrayCollection = [
   new ArrayObject([1,2,3]),
   [4,5,6],
];

foreach($arrayCollection as $item){
    $item[9]='blah';
}
print_r($arrayCollection);
/**
  Array
(
    [0] => ArrayObject Object
        (
            [storage:ArrayObject:private] => Array
                (
                    [0] => 1
                    [1] => 2
                    [2] => 3
                    [9] => blah
                )

        )

    [1] => Array
        (
            [0] => 4
            [1] => 5
            [2] => 6
        )

)
*/


<?php
$object = new ArrayObject([1,2,3]);
$array =  [4,5,6];

function append($array){
    $array[9]='bar';
}
append($array);
print_r($array);
/**
 Array
(
    [0] => 4
    [1] => 5
    [2] => 6
)
**/

append($object);
print_r($object);
/**
ArrayObject Object
(
    [storage:ArrayObject:private] => Array
        (
            [0] => 1
            [1] => 2
            [2] => 3
            [9] => bar
        )

)
**/