PHP, Unicode and ostriches.
Update: I've written a follow-up post that describes how I would like PHP's encoding support to be.
As the resident encoding geek on the Drupal team, it's usually my job to make sure Drupal handles encodings and Unicode correctly. I don't mind doing this, but PHP doesn't exactly make it easy. With the new search.module for Drupal 4.6 being Unicode-aware, this has become very obvious, as we've had to bump up the minimum required version of PHP to 4.3.3. The UTF-8 support in the Perl-compatible regular expressions in PHP 4.3.2 and earlier is completely broken. And now I've had a bug report about someone on PHP 4.3.8 who still had problems getting it to work.
I don't know why exactly, but as far as encodings go PHP is still in the stone-age. This is odd, as you'd expect a web-oriented scripting language to have excellent support for sharing and exchanging textual information. There is a multi-byte string extension available, but it's not available on 90% of PHP hosts out there, and it's more of a black-box library anyway: it does not present you your strings as Unicode character codepoints, but still as an array of bytes. Furthermore, if you actually enable the mbstring overrides, you lose the ability to work with bytes at will. Apparently, the PHP team still hasn't figured out that bytes and characters are not the same. The other extensions which deal with encodings (iconv, recode) are also unavailable on the majority of PHP installs out there.
This means that if you want to make a PHP application which supports any language and runs on the average PHP host out there, that there's only one option: use UTF-8 internally, and write your own functions for string truncation, email header encoding, validation, etc. Using UTF-8 ensures that you only have one encoding to worry about and because it's Unicode it is guaranteed to be able to represent any language. Of course, you will no longer be able to do something simple as upper/lowercasing a string, as these PHP functions don't take UTF-8 at all.
What PHP needs is Unicode string support in the core, along with a good library of useful functions for handling the very large Unicode character range efficiently. ASP, Perl, Python, Java all have it... for me, it's the only thing that would've made PHP5 worth to upgrade to.
It's as if the entire PHP team has stuck their head in the ground, hoping that all this Unicode stuff will somehow blow over. It won't.