There are two answers guaranteed to show up on nearly every PHP question on Stack Overflow - use the mysqli/PDO extension instead of the deprecated mysql one (to avoid SQL injection), and that you can't parse HTML with regular expressions.
With that in mind, my favorite two answers:
- [How PDO is still vulnerable to SQL injection attacks](http://stackoverflow.com/questions/134099/are-pdo-prepared-statements-sufficient-to-prevent-sql-injection/12202218#12202218) by Anthony Ferrara
- [Parsing HTML with regular expressions](http://stackoverflow.com/questions/4231382/regular-expression-pattern-not-matching-anywhere-in-string/4234491#4234491) by Tom Christiansen
I've posted a bunch of projects to Github in the last few months and haven't linked to them anywhere, so here's a quick roundup of recently notable projects:
- [lib_timezones](https://github.com/iamcal/lib_timezones) - A PHP and JS library to handle user-specified timezones properly, with reasonable client-side auto detection.
- [lib_classsify](https://github.com/iamcal/lib_classify) - A partial PHP port of Github's own [linguist](https://github.com/github/linguist) which can automatically detect the programming language, given a code fragment.
- [lib_autolink](https://github.com/iamcal/lib_autolink) - My old PHP URL-detection and linking library, updated to handle multiple protocols.
- [lib_solr_query](https://github.com/iamcal/lib_solr_query) - A PHP library to turn user-entered complex search queries into valid SOLR query syntax. Supports phrases, booleans, nesting, assertions and more.