注意:str_shuffle()内部使用rand()，不适合用于加密目的(例如生成随机密码)。你需要一个安全的随机数生成器。它也不允许字符重复。

还有一种方法。

UPDATED(现在这会生成任意长度的字符串):

function generateRandomString($length = 10) {
    return substr(str_shuffle(str_repeat($x='0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ', ceil($length/strlen($x)) )),1,$length);
}

echo  generateRandomString();  // OR: generateRandomString(24)

就是这样。：）

具有上面讨论的一些函数的类。

$options['numeric'] = true;
$options['uppercase'] = true;
$options['lowercase'] = true;
$new = new RandomString($options);

class RandomString
{
    /**
     * @var array
     */
    private $default = ['numeric' => true, 'uppercase' => true, 'lowercase' => true];

    /**
     * @var array
     */
    private $options;

    /**
     * array
     */
    private $whitelist = ['numeric', 'uppercase', 'lowercase'];

    /**
     * RandomString constructor.
     *
     * @param array $options
     */
    public function __construct(array $options = [])
    {
        $this->options = $this->default;
        if(!empty($options))
        {
            $options = array_intersect_key($options, array_flip($this->whitelist));
            if(empty($options))
            {
                $this->options = $this->default;
            }else
            {
                $this->options = $options;
            }
        }
    }

    /**
     * @return string
     */
    private function returnCharacters(){

        $options = $this->options;

        $numbers = '0123456789';
        $uppercase = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
        $lowercase = "abcdefghijklmnopqrstuvwxyz";

        $characters = '';
        if(isset($options['numeric']) && $options['numeric'] === true){
            $characters .= $numbers;
        }

        if(isset($options['uppercase']) && $options['uppercase'] === true){
            $characters .= $uppercase;
        }

        if(isset($options['lowercase']) && $options['lowercase'] === true){
            $characters .= $lowercase;
        }
        return $characters;
    }

    /**
     * @param $length
     * @param $quantity
     * @return string
     */
    public function randomString($length, $quantity) {

        $string = '';
        $characters = $this->returnCharacters();

        for ($j = 0; $j < $quantity; $j++) {
            for($i = 0; $i < $length; $i++){
                $string .= $characters[mt_rand(0, strlen($characters) - 1)];
            }
            $string .= "\n";
        }
        return $string;
    }

    /**
     * @return array
     */
    public function getOptions()
    {
        return $this->options;
    }

    /**
     * @return mixed
     */
    public function getWhitelist()
    {
        return $this->whitelist;
    }

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;
}

我已经测试了那里最流行的函数的性能，在我的盒子上生成1 000 000个32个符号的字符串所需的时间是:

2.5 $s = substr(str_shuffle(str_repeat($x='0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ', ceil($length/strlen($x)) )),1,32);
1.9 $s = base64_encode(openssl_random_pseudo_bytes(24));
1.68 $s = bin2hex(openssl_random_pseudo_bytes(16));
0.63 $s = base64_encode(random_bytes(24));
0.62 $s = bin2hex(random_bytes(16));
0.37 $s = substr(md5(rand()), 0, 32);
0.37 $s = substr(md5(mt_rand()), 0, 32);

请注意，它到底有多长并不重要，重要的是哪个更慢，哪个更快，因此您可以根据您的要求进行选择，包括密码准备等。

如果需要小于32个字符的字符串，则在MD5周围添加substr()以保证准确性。

为了回答:字符串没有被连接，而是被覆盖，函数的结果没有被存储。

只使用PHP本地函数的参数化一行程序，从PHP 5.1.0开始工作

str_shuffle(implode('', (array_intersect_key(($map =  array_map('chr', array_merge(array_map('mt_rand', array_fill(0, $length = 25, 48), array_fill(0,$length,57)),array_map('mt_rand', array_fill(0, $length, 65), array_fill(0,$length,90)),array_map('mt_rand', array_fill(0, $length, 97), array_fill(0,$length,122))))), array_flip($keys = array_rand($map, $length))))))

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;
?>