如果您非常害怕输入字母表中的字母，不关心字符串安全性，并且只对字母感兴趣，那么这里有一个简单的解决方案

$alphabets = range ("a", "z");
shuffle ($alphabets);
$randomString = substr(implode ("", $alphabets), 3, 17); // adjust according to desired length 

一个完整的解决方案(课程加测试)，部分基于上面的一些建议…

class TokenFactory
{
    private const LENGTH = 12;
    private const ALLOWED = '123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNPQRSTUVWXYZ~!@#$%^&*{}';
    private const MIN_NUMBER_OF_DIGITS = 1;
    private const MIN_NUMBER_OF_CAPS = 1;
    private const MIN_NUMBER_OF_SPECIALS = 1;
    private const MIN_NUMBER_OF_LETTERS = 1;

    /**
     * @return string
     * @throws \Exception
     */
    public function make(): string
    {
        $pass = $this->generateToken();

        if ($this->isTokenValid($pass)) {
            return $pass;
        } else {
            return $this->make();
        }
    }

    /**
     * @return string
     * @throws \Exception
     */
    private function generateToken(): string
    {
        $allowedCharacters = self::ALLOWED;
        $token              = '';
        $max               = mb_strlen($allowedCharacters, '8bit') - 1;
        for ($i = 0; $i < self::LENGTH; ++$i) {
            $token .= $allowedCharacters[random_int(0, $max)];
        }
        return $token;
    }

    /**
     * @param $token
     * @return bool
     */
    private function isTokenValid($token): bool
    {
        $numberOfDigits   = preg_match_all("/[0-9]/", $token);
        $numberOfCaps     = preg_match_all("/[A-Z]/", $token);
        $numberOfSpecials = preg_match_all("/[~!@#\$%^&*{}]/", $token);
        $numberOfLetters  = preg_match_all("/[a-z]/", $token);

        return
            $numberOfDigits > self::MIN_NUMBER_OF_DIGITS
            && $numberOfCaps > self::MIN_NUMBER_OF_CAPS
            && $numberOfSpecials > self::MIN_NUMBER_OF_SPECIALS
            && $numberOfLetters > self::MIN_NUMBER_OF_LETTERS
            ;
    }
}

class TokenFactoryTest
{
    public function test_correct_syntax()
    {
        /**
         * Arrange
         */
        $length = 12;
        $numberOfChecks = 1000;

        /**
         * Act & Assert
         */
        $class = new TokenFactory();

        $i = 0;
        while ($i < $numberOfChecks) {
            $generatedToken = $class->make();

            $numberOfDigits = preg_match_all( "/[0-9]/", $generatedToken );
            $numberOfCaps = preg_match_all( "/[A-Z]/", $generatedToken );
            $numberOfSpecials   = preg_match_all("/[~!@#\$%^&*{}]/", $generatedToken);
            $numberOfLetters   = preg_match_all("/[a-z]/", $generatedToken);

            Assert::assertEquals($length, strlen($generatedToken));
            Assert::assertTrue($numberOfDigits >= 1, 'Digit error: ' . $generatedToken);
            Assert::assertTrue($numberOfCaps >= 1, 'Caps error: ' . $generatedToken);
            Assert::assertTrue($numberOfSpecials >= 1, 'Specials error: ' . $generatedToken);
            Assert::assertTrue($numberOfLetters >= 1, 'Letters error: ' . $generatedToken);

            $i++;
        }
    }
}

顺便说一句，请确保在适合您需要的地方捕获该异常!

具体回答这个问题，有两个问题:

当你回显$randstring时，它不在作用域内。 字符在循环中没有连接在一起。

以下是更正后的代码片段:

function generateRandomString($length = 10) {
    $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $charactersLength = strlen($characters);
    $randomString = '';
    for ($i = 0; $i < $length; $i++) {
        $randomString .= $characters[random_int(0, $charactersLength - 1)];
    }
    return $randomString;
}

用下面的调用输出随机字符串:

// Echo the random string.
// Optionally, you can give it a desired string length.
echo generateRandomString();

请注意，这个答案的以前版本使用rand()而不是random_int()，因此生成可预测的随机字符串。因此，根据这个答案的建议，它被更改为更安全。

如果你需要大量无重复的随机字符串，这里有一个经过小修改的递归代码:

function generateRandomString($length = 10)
{
    global $generatedStrings;
    $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $randomString = '';
    for ($i = 0; $i < $length; $i++) {
        $randomString .= $characters[rand(0, strlen($characters) - 1)];
    }
    if (isset($generatedStrings[$randomString])) {
        $randomString = generateRandomString($length);
    }
    $generatedStrings[$randomString] = $randomString;
    return $randomString;
}

$generatedStrings = array();

foreach(range(1, 100000) as $num) {
    echo "\n".$num."\t : ".generateRandomString();
}

最后，我找到了一个解决方案，以获得随机和唯一的值。

我的解决方案是:

substr(md5(time()), 0, 12)

Time总是返回一个时间戳，并且总是唯一的。您可以将其与MD5一起使用以使其更好。

递归解决方案:

public static function _random(string $set , int $length): string
{
    $setLength = strlen($set);
    $randomKey = random_int(0, $setLength - 1);

    $firstPiece = substr($set, 0, $randomKey);
    $secondPiece = substr($set, $randomKey, $setLength - $randomKey);

    $removedCharacter = $firstPiece[strlen($firstPiece) - 1] ?? null;
    if(null === $removedCharacter || $length === 0) {
        return '';
    }
    $firstPieceWithoutTheLastChar = substr($firstPiece, 0, -1);

    return $removedCharacter . self::_random($firstPieceWithoutTheLastChar . $secondPiece, $length - 1);
}

不错的表现，https://3v4l.org/aXaJ6/perf