无法获取$ POST的值[关闭]

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            <div class="grid--cell mb0 mt8">Closed <span title="2012-08-01 15:43:07Z" class="relativetime">7 years ago</span>.</div>
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hello guys i have this problem i can't compare this two strings

<?php
 $dir = '/var/www/devData/test';

 // create new directory with 777 permissions if it does not exist yet
 // owner will be the user/group the PHP script is run under
 if ( !file_exists($dir) ) {
  mkdir ($dir, 0777);
 }



         $flag = 0;
         $stringData = $_POST['data']; // echo = null
         $file = "/var/www/devData/test/ciao.txt"; 
         $fh = fopen($file, 'a+') or die("can't open file");

         while(!feof($fh)) {
                    $theData = fgets($fh, filesize($file));
                    array_push($arr,$theData);
                    $i++;
         }  
         for($j=0; $j<count($arr); $j++) 
         if($stringData == $arr[j]){ // is the problem 
            $flag = 1;
         }
         if($flag = 0){
            fwrite($fh, $stringData); // fwrite works perfectly even if i try to print $string the result is null 
         }
         fclose($fh); 



 ?>

Someone can explain me how can resolve the problem? The goal of this script is avoid the user to write two times the same things

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