你可以试试这个:

<?php
    function random($len){

        $char = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';

        // ----------------------------------------------
        // Number of possible combinations
        // ----------------------------------------------
        $pos = strlen($char);
        $pos = pow($pos, $len);
        echo $pos.'<br>';
        // ----------------------------------------------

        $total = strlen($char)-1;
        $text = "";

        for ($i=0; $i<$len; $i++){
            $text = $text.$char[rand(0, $total)];
        }
        return $text;
    }

    $string = random(15);
    echo $string;
?>

您也可以准时使用md5，但要小心。

您需要使用microtime()而不是time()函数，因为如果多个线程在同一秒内运行，则需要为所有线程获取不同的字符串。

<?php

    $string = md5(microtime());
    echo $string;
?>

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!

function randomString() {
       return md5(rand(100, 200));
}

$pass = "";
for ($i=0;$i<12;++$i)
  $pass.= ($r=mt_rand(0,35))<26?chr(ord('a')+$r):chr(ord('0')+$r-26);

以前的答案会生成不安全或难以输入的密码。

这是安全的，并且提供了用户更有可能实际使用的密码，而不是因为一些薄弱的东西而被丢弃。

// NOTE: On PHP 5.x you will need to install https://github.com/paragonie/random_compat

/**
 * Generate a password that can easily be typed by users.
 *
 * By default, this will sacrifice strength by skipping characters that can cause
 * confusion. Set $allowAmbiguous to allow these characters.
 */
static public function generatePassword($length=12, $mixedCase=true, $numericCount=2, $symbolCount=1, $allowAmbiguous=false, $allowRepeatingCharacters=false)
{
  // sanity check to prevent endless loop
  if ($numericCount + $symbolCount > $length) {
    throw new \Exception('generatePassword(): $numericCount + $symbolCount are too high');
  }

  // generate a basic password with just alphabetic characters
  $chars  = 'qwertyupasdfghjkzxcvbnm';
  if ($mixedCase) {
    $chars .= 'QWERTYUPASDFGHJKZXCVBNML';
  }
  if ($allowAmbiguous) {
    $chars .= 'iol';
    if ($mixedCase) {
      $chars .= 'IO';
    }
  }

  $password = '';
  foreach (range(1, $length) as $index) {
    $char = $chars[random_int(0, strlen($chars) - 1)];

    if (!$allowRepeatingCharacters) {
      while ($char == substr($password, -1)) {
        $char = $chars[random_int(0, strlen($chars) - 1)];
      }
    }

    $password .= $char;
  }


  // add numeric characters
  $takenSubstitutionIndexes = [];

  if ($numericCount > 0) {
    $chars = '23456789';
    if ($allowAmbiguous) {
      $chars .= '10';
    }

    foreach (range(1, $numericCount) as $_) {
      $index = random_int(0, strlen($password) - 1);
      while (in_array($index, $takenSubstitutionIndexes)) {
        $index = random_int(0, strlen($password) - 1);
      }

      $char = $chars[random_int(0, strlen($chars) - 1)];
      if (!$allowRepeatingCharacters) {
        while (substr($password, $index - 1, 1) == $char || substr($password, $index + 1, 1) == $char) {
          $char = $chars[random_int(0, strlen($chars) - 1)];
        }
      }

      $password[$index] = $char;
      $takenSubstitutionIndexes[] = $index;
    }
  }

  // add symbols
  $chars = '!@#$%&*=+?';
  if ($allowAmbiguous) {
    $chars .= '^~-_()[{]};:|\\/,.\'"`<>';
  }

  if ($symbolCount > 0) {
    foreach (range(1, $symbolCount) as $_) {
      $index = random_int(0, strlen($password) - 1);
      while (in_array($index, $takenSubstitutionIndexes)) {
        $index = random_int(0, strlen($password) - 1);
      }

      $char = $chars[random_int(0, strlen($chars) - 1)];
      if (!$allowRepeatingCharacters) {
        while (substr($password, $index - 1, 1) == $char || substr($password, $index + 1, 1) == $char) {
          $char = $chars[random_int(0, strlen($chars) - 1)];
        }
      }

      $password[$index] = $char;
      $takenSubstitutionIndexes[] = $index;
    }
  }

  return $password;
}

这将创建一个20个字符的十六进制字符串:

$string = bin2hex(openssl_random_pseudo_bytes(10)); // 20 chars

在PHP 7 (random_bytes())中:

$string = base64_encode(random_bytes(10)); // ~14 characters, includes /=+
// or
$string = substr(str_replace(['+', '/', '='], '', base64_encode(random_bytes(32))), 0, 32); // 32 characters, without /=+
// or
$string = bin2hex(random_bytes(10)); // 20 characters, only 0-9a-f