是否可以在连续的ajax请求中为php变量分配一次值？

Question

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

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.

 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 && xmlhttp.status==200)
       {
          document.getElementById('read').innerHTML = xmlhttp.responseText;
       } 
    }
    xmlhttp.open("GET","demo.php",true);
    xmlhttp.send();
 }

My php file

<?php 
    $a = fileattime("filenam");
    $b = fileattime("filenam");
    if($b > $a )
    {
     echo "update contents";
     $a = $ b;
    }

?>

I thought to store the last modified time of file in $a variable and compare the value with $b variable but here $a will be overwritten with the new value for every ajax request. I want $a to be assigned a value only once, I mean for the first ajax request then later on compare it with $b value and if $b is greater than $a then update the contents to user then fill up $a with $b

but this is where i stucked at. Any help is greatly appreciated.Thanks

Answer 1

Try creating a session, and storing the information in a session variable.

http://us3.php.net/manual/en/reserved.variables.session.php

If you do that, you'll have a persistent value that you can use to test against to see the last modified / updated value.

Be sure to do session_start() at the beginning of the script.

You can have it look something like this.. JS:

// standard Ajax stuff
xmlhttp.open("GET", "demo.php?request=1", true);
xmlhttp.send();

PHP:

<?php
    session_start();
    if($_GET['request'] == 1) $_SESSION['modified_since'] = fileattime('filename');
    $a = $_SESSION['modified_since'];
    $b = fileattime("filename");
    if($b > $a) {
        echo "Update Contents";
        $_SESSION['modified_since'] = $b;
    }
?>