PHP删除或保存在一个字符串中的重复文字

class StringClean{
      
    public function keep_nth($string, $subject, $replacement='|CLEANUP|',  $nth=1){
          
        $contents      = $this->replace_nth($string, $subject,$replacement,  $nth);
  
        $contents      = str_replace($replacement,$string,$contents);
          
        return $contents;
    }
      
    public function replace_nth($string, $subject, $replacement='', $nth=1){
          
        $regExPattern = "/".preg_quote($string, '/')."/";
          
        return preg_replace_callback($regExPattern,
            function($found) use (&$regExPattern, &$replacement, &$nth) {
                    $nth--;
                    if ($nth==0) return preg_replace($regExPattern, $replacement, reset($found) );
                    return reset($found);
            }, $subject,$nth  );
    }
      
}

//呵呵以网站域名为例（顺便打个小广告）
$contents ='http://www.php.cn';
  
$StringRemoveDuplicates = new StringClean;
  
$contents = $StringRemoveDuplicates->keep_nth('codepearl', $contents,"", 2); 
  
var_dump($contents );  
//输出：http://www.php.cn