title: In PHP fopen function， what's the difference between w and w+ mode?
body: <p>I have read the manaul, and knew that w+ write and read, w only write. But how to understand it? </p>
  
  <pre><code>$file_name =  $_SERVER['DOCUMENT_ROOT']."/../data/test.txt";  //file is exist
  $fp = fopen($file_name, 'w+');
  fwrite($fp, 'hello, world!');
  $len = filesize($file_name); 
  echo $len; //13
  $contents = fread($fp, $len);
  var_dump($contents);//string(0) ""    why?
  fclose($fp);
  </code></pre>