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

echo generateRandomString(20);


/**
 * 随机生成字符串
 * @param int $length
 * @return null|string
 */
private static function getRandChar($length = 8){
  $str = null;
  $strPol = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789abcdefghijklmnopqrstuvwxyz";
  $max = strlen($strPol)-1;
 
  for($i=0;$i<$length;$i++){
    $str.=$strPol[rand(0,$max)]; //rand($min,$max)生成介于min和max两个数之间的一个随机整数
  }
 
  return $str;
}