title: Why php json_encode encodes two dimensional array but outputs pure array
body: <pre><code>$my_array = array(
          0 =&gt; array(
                  1,
                  2,
                  3,
              ),
          1 =&gt; array(
                  1,
                  2,
                  3,
              ),
          2 =&gt; array(
                  1,
                  2,
                  3,
              ),
      );
  echo json_encode($my_array);
  </code></pre>
  
  <p>Result is this:</p>
  
  <pre><code>[[1,2,3],[1,2,3],[1,2,3]]
  </code></pre>
  
  <p>I thought the output should be a string of json,but here outputs a pure array,why?In other words,the result should be quoted,but why this is not.</p>