title: is it possible to assign a value only once for php variable in continuous ajax request?
body: <p>Well I have some simple text file, I am trying to read that file contents using php and update it to the user.I was setting up the ajax request for every 2 seconds but the problem is the file wont be updated for every 2 seconds may be 15 or 30 or 1 minute long.So i thought of doing this</p>
  
  <p>check the last modified time of file and if it is different then only update it to the user but here where I am stucked at.</p>
  
  <pre><code> Javascript functions
  
   function read()
   {
      read_text();
      t=setTimeout("read()",200);
   }
  
   function read_text()
   {
       var xmlhttp;
       if (window.XMLHttpRequest)
       {// code for IE7+, Firefox, Chrome, Opera, Safari
         xmlhttp=new XMLHttpRequest();
       }
       else
      {// code for IE6, IE5
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
      }
      xmlhttp.onreadystatechange=function()
      {
          if (xmlhttp.readyState==4 &amp;&amp; xmlhttp.status==200)
         {
            document.getElementById('read').innerHTML = xmlhttp.responseText;
         } 
      }
      xmlhttp.open("GET","demo.php",true);
      xmlhttp.send();
   }
  </code></pre>
  
  <p><strong>My php file</strong></p>
  
  <pre><code>&lt;?php 
      $a = fileattime("filenam");
      $b = fileattime("filenam");
      if($b &gt; $a )
      {
       echo "update contents";
       $a = $ b;
      }
  
  ?&gt;
  </code></pre>
  
  <p>I thought to store the last modified time of file in <code>$a</code> variable and compare the value with <code>$b</code> variable but here <code>$a</code> will be overwritten with the new value for every ajax request. I want <code>$a</code> to be assigned a value only once, I mean for the first ajax request then later on compare it with <code>$b</code> value and if <code>$b</code> is greater than <code>$a</code> then update the contents to user then fill up <code>$a</code> with <code>$b</code></p>
  
  <p>but this is where i stucked at. Any help is greatly appreciated.Thanks</p>