In this post on the Dokeos blog, there's a comparison of the mbstring function and the iconv library as it pertains to their use on multi-byte strings.

I was wondering today why use mbstring rather than iconv in Dokeos, and honestly I didn't remember exactly why I had chosen mbstring in the past, but finding information about the *differences* between the two. [...] Searching a bit more, I found a PPT presentation from Carlos Hoyos on Google.

Essentially, it boils down to how the library is integrated - mbstring is bundled and iconv is pulled from an external source. So, if you're looking for maximum portability, he recommends mbstring.