An Issue in PHP Multidimensional Array

When assing an element of multidimensional array in PHP, such as:

$arr = array(array());
$idx = 0;
$arr['Key_1'][$idx] = $something;

indeed, there would be automatically generate another element in front:

$arr[0] = '';

As a result, there are two elements:

$arr[0] = '';
$arr['Key_1'][0] = 'something';

However, when keys do include [0], it would be no such issue. To fix up the problem, following code needs to be added before return the array:

unset($arr[0]);