Maybe one nice resource for blogging ideas is reacting to other blogs discussing a problem, theoretical or not, that someone has tried to solve via PHP or some other language. I'm thinking of this one: Programming images on the fly as an example, where he says
in reply to The Enlightened Perl Iron Man Competition
If I were scripting this to run as a local application, I'd probably use Bash, a touch of AWK, and ImageMagick. PERL might even be a better choice, but I'm not all that comfortable in that language. Since I want a web app, I decided on PHP as the thin glue to hold things together. In this case, how Perl could do it better or differently or whatever.
Commenting systems are great for technical blogs. Often I'll bookmark a page because people post solutions or various solutions to some blog post in the comments, or watch-outs and getcha's. You have to do more work filtering spam, but comments can make a blog excellent and attractive to viewers.