What is the reason of not getting the content of post[$json] after decode it?

when I cal the JSON through POST json string  come but decode function not work but when I tried the same JSON without POST it worked why? and how it can be worked with post[$json]?

What I have tried:

<pre>here is the code

      $json=$_POST['logjson'];
      $content = json_decode($json, true);
      file_put_contents('testcontent.txt',$json);//here i get the json string which is'[{"Count":"aaa","Er":"1","Ve":"1","Ie":"12","Dae":"15-7-22","Woe":"15-7-21"}]'
      $e = $content['Count'];
      file_put_contents('test.txt',$e);//here i get null
but when i give the string direct to json like
 $json='[{"Count":"aaa","Er":"1","Ve":"1","Ie":"12","Dae":"15-7-22","Woe":"15-7-21"}]';
now  all code is worked.