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My solution depends on two things to hold true for a blog entry: 1) the first paragraph is sufficient to generate the entire description meta tag, and 2) the tags/categories for a blog entry are sufficient to generate the keywords meta tag.
The result is something like this (using this post as an example):
So lets see how to do this. First the fp-includes/core/core.entry.php file needs to be updated. Specifically the entry_parse() function. My new code is added just before the last two lines of this method, as shown below. What the code does is look for the first new line character in the entry content, then it pulls out the substring from the beginning of the content up to the new line i.e. the first paragraph. After this the bbcode and any HTML tags are stripped out. The final string is then set as the meta_description array element for the entry.
Now that the meta description is generated, it has to be added to the theme template. I added this to my header.tpl file like this:
There is a small catch here. Since this information is generated for an entry only, it will only work inside of an entry block in the template. So my header.tpl file is included like this this:
This can be done in the single.tpl or comments.tpl files (if comments are enabled).
The bonus of this approach is that the meta tags are not generated for pages that are not a blog entry page.
The same approach can be followed for static pages. In this case the core.static.php file needs to be updated by modifying the static_parse() function in a similar fashion to the code above.