/**
 * Test shuffleString
 */
function testShuffleString() {
    $shuffled = shuffleString("ĄęźćÓ");
    if (\mb_strlen($shuffled) != 5) {
        throw new \UnexpectedValueException("Invalid count of characters");
    }
    if ($shuffled == "ĄęźćÓ") {
        throw new \UnexpectedValueException("The same string");
    }
    foreach (["Ą", "ę", "ź", "ć", "Ó"] as $char) {
        if (\mb_strpos($shuffled, $char) === false) {
            throw new \UnexpectedValueException("Character not found");
        }
    }
}

/**
 * Shuffle string
 *
 * @param $stringValue String to shuffle
 * @param string $startWith Shuffle $stringValue and append to $startWith
 * @return string Shuffled string
 * @author Krzysztof Piasecki<krzysiekpiasecki@gmail.com>
 */
function shuffleString($stringValue, $startWith = "") {
    $range = \range(0, \mb_strlen($stringValue));
    shuffle($range);
    foreach($range as $index) {
        $startWith .= \mb_substr($stringValue, $index, 1);
    }
    return $startWith;
};

testShuffleString();

echo shuffleString("Hello"); // > 'elHol' (something like this)
echo shuffleString("World!", "Hello "); // > 'Hello do!lrW' (something like this)