I've been struggling with some charset issues, I first had to modify the index.php page to get logins to work, and then all emails were showing up as (no subject), and emails would appear as completely blank.
After a fair bit of debugging, I noticed that my libiconv libraries dated back to 2001. I downloaded, compiled, and installed the current libiconv-1.12 and magically all was well.
So. If you're experience strange issues like this, it might not be a problem with Roundcube or PHP at all - you might simply have out-of-date libraries.