Full text feed will give the complete content of your topic in contrast to summary of the content. Full text feed subscriber will not come to your website for reading the content, thus for a website owner it means less traffic. Most non-ecommerce website revenue depends upon advertisement which is affected by the number of visitors to website. So a website may even loose revenues with full text website ! Also with full text feed your content can be easily copied.
If full text feed have so many problems then why do famous bloggers still prefer Full text feed over Summary feed ? Answer is simple, because its good for Users and publishers (website owners). If your user has subscribed to your Feed this means that he is interested in your website and is willing to be your loyal reader. Trust me, its annoying to move from feed reader to your website just to find the content was not interesting. Moreover if a user likes your website content then there are other ways like comments, replies, pictures etc. to bring him back then just your content. As for the revenues issue, even the atom feeds can have their own advertisement thanks to feedbuner by Google.
The content needs to be escaped to show the entire content of the topic and we will use the <content> tag and not the <summary> tag. nl2br() is a PHP function which will convert a new line to <br/>. htmlspecialchars() is a PHP function to convert HTML tags to special character recognised by your browser and to avoid any feed error while generating the full text feed. The content tag will have the type declaration as html.
The PHP code would be modified to
// Since we will not be using the summary, so either delete or mark it as comment
// echo "<summary>".row['subtitle']."</summary>";
echo "<content type='html'>".htmlspecialchars(nl2br($row['content']))."</content>";
Well thats it, hope that switching to full text feed results in more happy subscribers for your website. ;-)