Return HTML or XML based on request in PHP

There's an existing website written in PHP. Originally only the website existed, but now an Android application is being built that would benefit from re-using some of the PHP logic.

The PHP site was structured such that there are many pages that perform an action, set success/error information in $_SESSION, and then redirect to a visual page without outputting any content themselves. For example, there's action_login.php:

The page accepts a username and password (from GET or POST variables), validates the credentials, sets success/failure messages in $_SESSION, and then redirects to the logged-in homepage on success or back to the login screen on failure. Let's call this behavior the "HTML response".

The Android application will need to call the same page but somehow tell it that it wants an "XML response" instead. When the page detects t开发者_运维知识库his, it will output success/error message in an XML format instead of putting them in $_SESSION and won't redirect. That's the idea anyway. This helps prevent duplicate code. I don't want to have action_login.php and action_login.xml.php floating around.

I've read that the Accept Header isn't reliable enough to use (see: Unacceptable Browser HTTP Accept Headers (Yes, You Safari and Internet Explorer)). My fallback solution is to POST xml=1 or use {url}?xml=1 for GET requests. Is there a better way?

No frameworks are being used, this is plain PHP.

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That's what the Accept Header is for. Have the Android request the page as application/xml and then check what was requested in your script. You might also be interested in mod_negotiation when using Apache. Or use WURFL to detect the UserAgent and serve as XML when Android.

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I'd go with the android app sending a cookie for every request (really I would prefer the Accept header, but with the problems you pointed out with webkit I understand your reluctance to do so). The cookie simplifies the code server-side to not have to check for $_GET['xml'] or $_POST['xml'], and if some android user shares an URL of your application and it had a ?xml=1, the user who opens this in a computer browser would receive XML instead of the normal web output.

I wouldn't rely on $_SESSION for mobile applications because users (or at least I do) on mobile platforms tend to open your app, play 5 minutes, put mobile on pocket and 2 hours later return to your app. Do you want to set a session lifetime so long?

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why not set a specific session for the app and then only set the header if the session is set something along the lines of

$_SESSION['app'] = "andriod app";

if ($_SESSION['app'] == "andriod app") {
  header..

not really sure how to implement this into an app as I've done really little work with apps but hope this helps your thought process