assertKeys.php

<?php

public function assertKeys($expected, $array)
{
    // we want to find if there are differences between the expected keys
    // and what the $array has provided. array_diff_key does the job but
    // it is incomplete because it is unidirectional:
    //
    // array_diff_key(['a', 'b'], []) !== array_diff_key([], ['a', 'b']);
    // []                             !== ['a', 'b']
    //
    // to fix this, we need to perform the operation in the reverse direction.
    // however, some keys may appear twice. so we merge the two array and then
    // find the unique values.
    //
    // why not union? a union "+" operation messes with the indices of an
    // array and will return unexpected values.
    $result1 = array_diff_key($expected, $array);
    $result2 = array_diff_key($array, $expected);
    $result = array_unique(array_merge($result1, $result2));

    if (count($result) > 0) {
        return $result;
    }

    return null;
}