function random_code($length = 8,$chars = null){
  if(empty($chars)){
    $chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
  }
  $count = strlen($chars) - 1;
  $code = '';
  while( strlen($code) < $length){
    $code .= substr($chars,rand(0,$count),1);
  }
  return $code;
}
echo random_code;//A1zYbN5X

function random_char($length = 8,$chars = null){
  if( empty($chars) ){
    $chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
  }
  $chars = str_shuffle($chars);
  $num = $length < strlen($chars) - 1 ? $length:str_len($chars) - 1;
  return substr($chars,0,$num);
}

function random_code_type($length = 8,$type = 'alpha-number'){
  $code_arr = array(
    'alpha' => 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ',
    'number'=> '0123456789',
    'sign'  => '#$%@*-_',
  );

  $type_arr = explode('-',$type);

  foreach($type_arr as $t){
    if( ! array_key_exists($t,$code_arr)){
      trigger_error("Can not generate type ($t) code");
    }
  }

  $chars = '';

  foreach($type_arr as $t){
    $chars .= $code_arr[$t];
  }
  $chars = str_shuffle($chars);
  $number = $length > strlen($chars) - 1 ? strlen($chars) - 1:$length;
  return substr($chars,0,$number);
}

echo random_code_type(8,"alpha-number-sign");#kXM*mC$S