/**
 * Flattens a multi-dimensional array into a single-dimensional array.
 *
 * @param array $array The array to flatten.
 * @return array The flattened array.
 */
function flattenArray(array $array): array
{
    $result = [];
    array_walk_recursive($array, function($item) use (&$result) {
        $result[] = $item;
    });
    return $result;
}

$multiDimensionalArray = [
    1,
    [2, 3],
    [4, [5, 6]],
    7,
    ['a', ['b', ['c']]]
];

$flatArray = flattenArray($multiDimensionalArray);

echo "Original multi-dimensional array:
";
print_r($multiDimensionalArray);
echo "
Flattened array:
";
print_r($flatArray);