Step 1: Find your content.
An RSS feed should probably list all the content items in your main section. For example, if you run a blog, you can have an RSS feed of all your latest posts. This blog does. If you run a small download site, consider a feed of your latest releases. Whatever content you choose, make sure it’s interesting, and will be updated regularly - aim for between 3-6 times a week, if you have a choice. Then just write out your basic database logic and get the data into an array. I’ll use $items for this example.

Step 2: Get your basic RSS structure.
RSS is an XML-based format, so all you really need is a set of XML tags as part of the RSS specification. RSS provides a number of optional tags, and being XML, it can be (and often is) extended with any other tag. For example, the Dublin Core metadata is a common inclusion. You can borrow the basic structure from any feed, including ours (load it up and right click > view source), but here’s a sample you can use:

<?xml version="1.0" encoding="UTF-8"?>
<rss version="2.0">
<channel>
  <title>My Blog Name</title>
  <link>http://example.com/blog</link>
  <description>Keeping webmasters up-to-date on technology.</description>
  <pubDate>Tue, 15 Apr 2008 18:00:00 +0000</pubDate>
  <item>
    <title>Story Title</title>
    <pubDate>Tue, 15 Apr 2008 18:00:00 +0000</pubDate>
    <link>http://example.com/my/story</link>
    <description>Today I saw a whale!</description>
  </item>
</channel>
</rss>

The easiest way to build your RSS feed in PHP is to leave all of that XML as plain text and only loop over the sections as needed (the <item> tags). The one gotcha here, as you’ve probably noticed, is the opening <?xml tag. This will probably clash with PHP, so instead echo that line out:

<?php echo "<?xml version=\"1.0\" encoding=\"UTF-8\"?>"; ?>

Then your structure is all in place. The item element is repeated for each story we have in our RSS feed; the date is RFC 2822 formatted (just use date("r")). We’ll use a simple foreach loop to put the content in:

<?php foreach ($items as $item) { ?>
  <item>
    <title><?=$item['title']?></title>
    <pubDate><?=date("r",$item['time'])?></pubDate>
    <link><?=$item['url']?></link>
    <description><?=$item['description']?></description>
  </item>
<?php } ?>

3. Serve it up!
To be recognised as an RSS feed, you’ll need to serve it as the right content type. Technically you should use application/rss+xml, but most browsers won’t recognise this. I find application/xhtml+xml works well, as does text/xml. The choice is yours, although IE7 seems to like text/xml better.

<?php header("Content-type: text/xml"); ?>

And we’re done!

Here’s a sample of the final product:

<?php header("Content-type: text/xml"); ?>
<?php echo "<?xml version=\"1.0\" encoding=\"UTF-8\"?>"; ?>
<rss version="2.0">
<channel>
  <title>My Blog Name</title>
  <link>http://example.com/blog</link>
  <description>Keeping webmasters up-to-date on technology.</description>
  <pubDate>Tue, 15 Apr 2008 18:00:00 +0000</pubDate>
<?php foreach ($items as $item) { ?>
  <item>
    <title><?=$item['title']?></title>
    <pubDate><?=date("r",$item['time'])?></pubDate>
    <link><?=$item['url']?></link>
    <description><?=$item['description']?></description>
  </item>
<?php } ?>
</channel>
</rss>