PHP and AJAX Caching Bug

Wow, this one “bugged” me for days. 

I have a PHP form that gets data from Oracle.  When I click the submit button, I want another page to process some script to keep a many-to-many table up to data.

For example:

<input type=submit onClick=submitDistricts($PUBID)>

The Javascript calls a GET

function submitDistricts(PUBID)
var xmlHttp = getXMLHttp();
    xmlHttp.onreadystatechange = function()
    if(xmlHttp.readyState == 4)
 xmlHttp.open(“GET”, “updateDistricts.php?PUBID=”+PUBID);

And this works pretty well.  Except that the call gets cached and I can only submit it once.  YIKES!!

How did I fix it?  Well ensure the GET is always unique.

Change the line from:

xmlHttp.open(“GET”, “updateDistricts.php?PUBID=”+PUBID);


xmlHttp.open(“GET”, “updateDistricts.php?PUBID=”+PUBID + ‘&’ + Math.random());

That’s it. Now my AJAX call to the database is always unique and has no caching…

That was a weird one!

2 Responses to “PHP and AJAX Caching Bug”

  1. 1 TMT
    March 12, 2009 at 11:08 pm

    Hi Gordon,


    Yeah, that’s a really tough one to track down; that’s bitten me too. The one problem is, the random number solutions (which just makes the browser’s cache think it’s a different document) will start cluttering up the cache with redundant copies of the same/similar data. Of course, that’s better than something not working at all, but it’s a side effect you might need to keep track of.

    More at http://en.wikipedia.org/wiki/XMLHttpRequest#Caching which also suggests putting a meta tag into the HTML (when applicable) to force the contents to expire. Of course, if it’s not HTML, that’s not an option.

  2. 2 Gordon
    March 13, 2009 at 1:13 pm

    Thanks Tomas,
    I went to that site and noticed the meta-tag workaround for caching, but I haven’t tried it yet:

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Gordon Luckett

Arrow Geomatics Inc's Gordon Luckett


gordon dot luckett at arrowgeo dot com 1-519-837-9500 (Arrow Geomatics Inc.)

Checkout MapGuide Guy’s Youtube Channel


Error: Twitter did not respond. Please wait a few minutes and refresh this page.


%d bloggers like this: