Functional\Exceptions\InvalidArgumentException::assertValidArrayKey PHP Method

    public static function assertValidArrayKey($key, $callee)
    {
        $keyTypes = ['NULL', 'string', 'integer', 'double', 'boolean'];
        $keyType = gettype($key);
        if (!in_array($keyType, $keyTypes, true)) {
            throw new static(sprintf('%s(): callback returned invalid array key of type "%s". Expected %4$s or %3$s', $callee, $keyType, array_pop($keyTypes), join(', ', $keyTypes)));
        }
    }

/**
 * Groups a collection by index returned by callback.
 *
 * @param Traversable|array $collection
 * @param callable $callback
 * @return array
 */
function group($collection, callable $callback)
{
    InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
    $groups = [];
    foreach ($collection as $index => $element) {
        $groupKey = $callback($element, $index, $collection);
        InvalidArgumentException::assertValidArrayKey($groupKey, __FUNCTION__);
        if (!isset($groups[$groupKey])) {
            $groups[$groupKey] = [];
        }
        $groups[$groupKey][$index] = $element;
    }
    return $groups;
}