Question

What I want to do is have a normal link for desktop browsers like so:

<div style="text-align: center;">
   <a href="http://db.tt/b6ecH2Dh">
      <img src="img/download.gif"/>
</div>

But when a iOS user clicks the link it should open a different link like this:

<div style="text-align: center;">
   <a href="cydia://package/bla">
       <img src="img/download.gif"/>
</div>

Could I do it with JavaScript or PHP? if so how?

EDIT
To do it with javascript, put this script INTO THE OF YOUR HTML DOC

<script type="text/javascript">
if (navigator.userAgent.match(/Mac OS X/i)) {
                document.getElementById('down').href="http://www.google.com";
        }
</script>

Answer 1

if((navigator.userAgent.match(/iPhone/i)) || (navigator.userAgent.match(/iPod/i))) 
{
     document.getElementById('download_link').href = "http://iphonedl";
}

Should work

http://davidwalsh.name/detect-iphone