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GD library is now bundled with the distribution and it is recommended to always use the bundled version
vpopmail and cybermut extensions are moved to PECL
several deprecated extensions (aspell, ccvs, cybercash, icap) and SAPIs (fastcgi, fhttpd) are removed
speed improvements in a variety of string functions
Apache2 filter is improved, but is still considered experimental (use with PHP in prefork and not worker (thread) model since many extensions based on external libraries are not thread safe)
various security fixes (imap, mysql, mcrypt, file upload, gd, etc)
new SAPI for embedding PHP in other applications (experimental)
much better test suite
significant improvements in dba, gd, pcntl, sybase, and xslt extensions
debug_backtrace() should help with debugging
error messages now contain URLs linking to pages describing the error or function in question
Zend Engine has some fixes and minor performance enhancements
and TONS of other fixes, updates, new functions, etc