addtooffset.php

<?php

class Foo
{
    public $data = array();
    
    public function addToOffset()
    {
        $args = func_get_args();
        $value = $args[0];
        array_shift($args);
        $element =& $this->data;
        foreach ($args as $offset) {
            if (isset($element[$offset]) && is_array($element[$offset])) {
                $element =& $element[$offset];
            } else {
                throw new \InvalidArgumentException("The specified path does not exsist or is not an array at " . $offset);
            }
        }
        $element = $value;
    }
}

$a = new Foo();
$a->data = array(
    'files' => array(
        array(
            'id' => 1
        ),
        array(
            'id' => 2
        )
    )
);

$a->addToOffset(array('error' => 'foo'), 'files');

var_dump($a->data);

gistfile1.txt

⎈ ⇢ php offset.php 
array(1) {
  'files' =>
  array(1) {
    'error' =>
    string(3) "foo"
  }
}