title: why back not load previous page and how to change url?
body: <p>suppose: i click 1.php then 2.php then 3.php then press back but it does not go to 2.php
  also url is not effected. I want to change url and support for back also, so it can be SEO friendly
   here is my code:</p>
  
  <pre><code>&lt;pre&gt;
  &lt;html&gt;
  &lt;head&gt;
      &lt;script type="text/javascript"&gt;
          function show(addr) {
              a = new XMLHttpRequest();
              a.onreadystatechange = function() {
                  if (a.readyState==4 &amp;&amp; a.status==200) {
                      document.getElementById("content").innerHTML=a.responseText;
                  }
              }
              a.open("GET",addr+".php",true); 
              a.send(); //
          }
      &lt;/script&gt;
  &lt;/head&gt;
  &lt;body&gt;
      &lt;a href="#" onClick="show('1')"&gt;Load 1.php into content&lt;/a&gt;
      &lt;a href="#" onClick="show('2')"&gt;Load 2.php into content&lt;/a&gt;
      &lt;a href="#" onClick="show('3')"&gt;Load 3.php into content&lt;/a&gt;
      &lt;a href="#" onClick="show('4')"&gt;Load 4.php into content&lt;/a&gt;
  
      &lt;!-- Load content here--&gt;
      &lt;div id="content"&gt;&lt;/div&gt;        
  &lt;/body&gt;
  &lt;/html&gt;
  &lt;/pre&gt;
  </code></pre>