The documentation for Differ::diff states that it accepts both arrays and strings as parameters:

/**
     * Returns the diff between two arrays or strings as string.
     *
     * @param array|string                            $from
     * @param array|string                            $to
     * @param LongestCommonSubsequenceCalculator|null $lcs
     *
     * @return string
     */
    public function diff($from, $to, LongestCommonSubsequenceCalculator $lcs = null): string

This is clearly not the case since Differ::validateDiffInput explicitly tries to cast everything to a string except arrays:

/**
     * Casts variable to string if it is not a string or array.
     *
     * @param mixed $input
     *
     * @return string
     */
    private function validateDiffInput($input): string
    {
        if (!\is_array($input) && !\is_string($input)) {
            return (string) $input;
        }
        return $input;
    }

The first fix is obvious, the documentation for Differ::diff needs to be updated. The second issue, is maybe we want to throw an InvalidArgumentException on an array instead of the much more opaque:

TypeError: Return value of SebastianBergmann\Diff\Differ::validateDiffInput() must be of the type string, array returned

Throwing InvalidArgumentException with an appropriate description would represent a better user experience.

If we agree, let me know, and I'll submit a PR with both fixes referencing this issue.