Anit spam filter based on counters like n (messages) per t (period) k times + protection from repeated images/text content via md5 hash (byte in case of image) comparison, also as additional params passed through to protect accidential repetition of text messages and images

AntiSpam.php

/**
 * $userId - int
 * $text - post, message etc. text - string
 * $img - image - resource usually $_FILES['image']['tmp_name']
 * $counterLimit - limit of n messages in t period - int
 * $countFails - limit of x failes after which user will be blocked - int
 * $addParams - ex.: array('period_same_counter'=>3, 'period_same_time'=>15*60); - array
 */
class AntiSpam {

  // ANTI-SPAM CONSTS
  const SPAM_TIME_LIMIT = 60, // NOT MORE THEN n SECONDS
          SPAM_COUNTER_LIMIT = 10, // NOT MORE THEN n MESSAGES PER TIME_LIMIT ^
          FAILS_LIMIT = 10, // NOT MORE THEN n FAILS PER COUNTER_LIMIT ^      
          SAME_MSGS_LIMIT = 200, // LIMIT TO BLOCK
          TEXT_NOT_CHECKED = 45, // MIN SYMBOLS TO CHECK
          SIMILARITY_THRESHOLD = 8, // AMOUNT OF SIMILAR BLOCKS PREV TO NEXT MSG
          SIMILAR_BLOCKS_MSG = 10, // FIND THAT AMOUNT OF BLOCKS
          INDEX_DIVISION = 10; // SEARCH INDEX DIVISION      
  
  public function isSpam($userId, $text, $img, $counterLimit = self::SPAM_COUNTER_LIMIT, 
          $countFails = true, $addParams = []) {

    $row = [];
    $row['same_counter'] = 0;
    $row['salt'] = 0;
    $sameForPeriod = false;
    $addSql = '';

    // create only if not exists
    $this->dbConn->exec("CREATE TABLE IF NOT EXISTS anti_spam("
            . "user_id INT UNSIGNED NOT NULL, "
            . "text VARCHAR(128) NOT NULL, "
            . "time_start INT UNSIGNED NOT NULL, "
            . "counter INT UNSIGNED NOT NULL, "
            . "failed MEDIUMINT UNSIGNED NOT NULL DEFAULT 0, "
            . "INDEX BTREE(user_id))ENGINE=MEMORY CHARSET utf8 COLLATE utf8_general_ci");    
    
    $stmt = $this->dbConn->prepare("SELECT time_start, text, counter, failed, salt, salt_img, "
            . "same_counter, period_same_counter, period_time_start "
            . "FROM anti_spam "
            . "WHERE user_id=:user_id", array(PDO::ATTR_CURSOR => PDO::CURSOR_FWDONLY));

    $stmt->execute(array(':user_id' => $userId));

    $row = $stmt->fetch(PDO::FETCH_ASSOC);

    $now = time();
    $period = $now - self::SPAM_TIME_LIMIT;
    $saltText = md5(str_replace(' ', '', $text));

    $image = '';
    $saltImg = uniqid();
    if (is_file($img)) {
      $image = $img;
      $saltImg = md5_file($image);
    }
//    echo $saltImg.' '.$image;  
    $text = trim($text);
//        print_r($row);

    if (empty($row)) {

      // insert if hasn`t been written/added yet
      $this->dbConn->exec("INSERT INTO anti_spam SET user_id=$userId, text='$text', time_start=$now, counter=0, salt='$saltText', "
              . "same_counter=0, period_same_counter=0, period_time_start=$now");

      $row['time_start'] = $now;
      $row['counter'] = 0;
      $row['failed'] = 0;
      $row['text'] = '';
      
      $row['period_time_start'] = $now;
      $row['period_same_counter'] = 0;
    }

    // check if same pics for period
    if (!empty($addParams)) {

      $periodSame = $now - $addParams['period_same_time'];

      if ($row['period_time_start'] < $periodSame) { // period lapsed
        $row['period_time_start'] = $now;
        $row['period_same_counter'] = 0;
      } else {
        if ((!empty($img) && (string) $row['salt_img'] === (string) $saltImg) || (!empty($text) && (string) $row['salt'] === (string) $saltText)) { // prev and curr are the same
          $row['period_same_counter'] += 1;
          if ($row['period_same_counter'] >= $addParams['period_same_counter']) {
            // more then N same in T period
            $sameForPeriod = true;
          }
        }
      }
    }

    $addSql = ", period_same_counter={$row['period_same_counter']}, period_time_start={$row['period_time_start']} ";

    // check if failed equals LIMIT immideatelly block user
    if (($row['failed'] > self::FAILS_LIMIT && $countFails === true) || $row['same_counter'] > self::SAME_MSGS_LIMIT) {
      // block user in groups
//      $this->dbConn->exec("UPDATE road_group_users SET removed=1, "
//              . "removed_by_moder=1 "
//              . "WHERE user_id=$userId");

      $spamIssue = 0;

      if ($row['failed'] > self::FAILS_LIMIT) {
        $spamIssue = 1; // failed N times X times in T interval
      } else if ($row['same_counter'] > self::SAME_MSGS_LIMIT) {
        $spamIssue = 2; // N same msgs
      }

      // block user in users table
      $this->dbConn->exec("UPDATE users SET blocked=1, spam_issue=$spamIssue, spam_text='$text' "
              . "WHERE id=$userId");

      // can block users in messages in block_users table
      // flush failed counter and same_counter to proceed msgs after unblocking this user
//      $this->dbConn->exec("UPDATE anti_spam SET failed=0, same_counter=0 WHERE user_id=$userId");

      return true;
    } else {

      // same msgs by salt md5 and prev->next msg seekg
      if ((!empty($text) && (string) $row['salt'] === (string) $saltText) 
              || (!empty($img) && (string) $row['salt_img'] === (string) $saltImg)) {
        $row['same_counter'] += 2;
      } else if ($row['same_counter'] > 0) {
        // we should count down one by one, because hackers can send 1/1 1/2 1/n msgs                 
        $row['same_counter'] -= 1;
      }
      
      $similarity = $this->checkSimilarity($row['text'], $text); // comper prev to next msgs by blocks
      if ($similarity>self::SIMILARITY_THRESHOLD) { // avoiding upper -1
        $row['same_counter'] += 3;
      }
      
      $wordsIndex = $this->spamWordsIndex($text);
      
      if ($wordsIndex>=self::INDEX_DIVISION) {                
        $row['same_counter'] += round($wordsIndex/self::INDEX_DIVISION);            
      }
      
      // if less then period ago - update till now, update counter=0, and failed-1 if it is more then 0 
      if ($row['time_start'] < $period) {
        $row['time_start'] = $now;
        $row['counter'] = 0;
        if ($row['failed'] > 0)
          $row['failed'] = $row['failed'] - 1;
      } else {
        // if period hasn`t been lapsed, then counter+1 and if it is more then COUNTER_LIMIT - failed + 1        
        $row['counter'] = $row['counter'] + 1;
        if ($row['counter'] >= $counterLimit) {
          $row['counter'] = 0; // reset because of failed increase
          $row['failed'] = $row['failed'] + 1;
          // update data 
          $this->dbConn->exec("UPDATE anti_spam SET text='$text', "
                  . "time_start={$row['time_start']}, counter={$row['counter']}, "
                  . "failed={$row['failed']}, salt='$saltText', salt_img='$saltImg', same_counter={$row['same_counter']} $addSql "
                  . "WHERE user_id=$userId");
          return true;
        }
      }
    }

//    echo $row['same_counter'].' '.$row['salt_img'].' '.$saltImg;
    $sql = "UPDATE anti_spam SET text='$text', "
            . "time_start={$row['time_start']}, counter={$row['counter']}, "
            . "failed={$row['failed']}, salt='$saltText', salt_img='$saltImg', same_counter={$row['same_counter']} $addSql "
            . "WHERE user_id=$userId";

//    echo $sql;
    // update data 
    $this->dbConn->exec($sql);

    if ($sameForPeriod)
      return true;

//    if (mb_strlen($text, 'utf-8') > self::TEXT_NOT_CHECKED && $this->isSameSpam($userId, $saltText, $saltImg, $text))
//      return true;

    return false; // not a spam - ok
  }
  
}

Examples:

$userId = 123;
$msgText = 'some text goes here';
// usually U need to check image before upload
$tmpImage = $_FILES['image']['image_tmp'];
// not more then n messages per SPAM_TIME_LIMIT till k fails
$counterLimit = 9;
$countFails = 3;
// means not more then n same messages per t period 
// may be needed in situations where users accidentally post the same content
$addParams = array('period_same_counter' => 1, 'period_same_time' => 5 * 60);

// usage 
$antiSpam = new AntiSpam();
if ($antiSpam->isSpam($userId, $msgText, $tmpImage, $counterLimit, $countFails, $addParams)) {

  // send, set message, post, comment

}

// this time method returns false - text and image are the same
if ($antiSpam->isSpam($userId, $msgText, $tmpImage, $counterLimit, $countFails, $addParams)) {

  // send, set message, post, comment

}

!!! U`ll need a users table with at least 3 columns for blocking !!!

1. blocked
2. spam_issue
3. spam_text

Or U can change this block of code to provide Your own table fields.

      // block user in users table
      $this->dbConn->exec("UPDATE users SET blocked=1, spam_issue=$spamIssue, spam_text='$text' "
              . "WHERE id=$userId");