From php.net's PHP5 change log:
Version 5.0.0 Beta 1
29-Jun-2003
* Switch to using Zend Engine 2, which includes numerous engine level improvements. A full list is available at http://www.php.net/zend-engine-2.php.
* The SQLite (http://www.hwaci.com/sw/sqlite/) extension is now bundled and enabled by default. (Wez, Marcus, Tal)
* Improved the speed of internal functions that use callbacks by 40% due to a new internal fast_call_user_function() function. (Sterling)
* Completely Overhauled XML support (Rob, Sterling, Chregu, Marcus)
o Brand new Simplexml extension
o New DOM extension
o New XSL extension
o Moved the old DOM-XML and XSLT extensions to PECL
o ext/xml can now use both libxml2 and expat to parse XML
o Removed bundled expat
* Removed the bundled MySQL client library. (Sterling)
* New php.ini options:
o "session.hash_function" and "session.hash_bits_per_character". (Sascha)
o "mail.force_extra_paramaters". (Derick)
o "register_long_arrays". (Zeev)
* Improved the streams support: (Wez, Sara, Ilia)
o Improved performance of readfile(), fpassthru() and some internal streams operations under Win32.
o stream_socket_client() - similar to fsockopen(), but more powerful.
o stream_socket_server() - Creates a server socket.
o stream_socket_accept() - Accept a client connection.
o stream_socket_get_name() - Get local or remote name of socket.
o stream_copy_to_stream()
o stream_get_line() - Reads either the specified number of bytes or until the ending string is found.
o Added context property to userspace streams object.
o Added generic crypto interface for streams (supports dynamic loading of OpenSSL)
o Added lightweight streaming input abstraction to the Zend Engine scanners to provide uniform support for include()'ing data from PHP streams across all platforms.
o Added 'string.base64' stream filter.
o Renamed stream_register_wrapper() to stream_wrapper_register().
o Added "ftp://" wrapper support to opendir(), stat() and unlink().
o Added context options 'method', 'header' and 'content' for "http://" fopen wrapper.
* Improved the GD extension: (Pierre-Alain Joye, Ilia)
o imagefilter() - Apply different filters to image. (Only available with bundled GD library)
o Antialiased drawing support:
+ imageantialias() - (de)active antialias
+ imageline() and imagepolygon() antialias support
* Changed the length parameter in fgetss() to be optional. (Moriyoshi)
* Changed ini parser to allow for handling of quoted multi-line values. (Ilia)
* Changed get_extension_funcs() to return list of the built-in Zend Engine functions if "zend" is specified as the module name. (Ilia)
* Changed array_search() to accept also objects as a needle. (Moriyoshi)
* Changed ext/mcrypt to require libmcrypt version 2.5.6 or greater. (Derick)
* Changed uniqid() parameters to be optional and allow any prefix length. (Marcus)
* Added new iconv functions. (Moriyoshi)
o iconv_strlen()
o iconv_substr()
o iconv_strpos()
o iconv_strrpos()
o iconv_mime_decode()
o iconv_mime_encode()
* Added misc. new functions:
o ldap_sasl_bind(). (peter_c60@hotmail.com, Jani)
o imap_getacl(). (Dan, Holger Burbach)
o file_put_contents(). (Sterling)
o proc_nice() - Changes priority of the current process. (Ilia)
o pcntl_getpriority() and pcntl_setpriority(). (Ilia)
o idate(), date_sunrise() and date_sunset(). (Moshe Doron)
o strpbrk() - Searches a string for a list of characters. (Ilia)
o get_headers() - Returns headers sent by the server of the specified URL. (Ilia)
o str_split() - Breaks down a string into an array of elements based on length. (Ilia)
o array_walk_recursive(). (Ilia)
o array_combine(). (Andrey)
* Added optional parameter to get_browser() to make it return an array. (Jay)
* Added optional parameter to openssl_sign() to specify the hashing algorithm.(scott@planetscott.ca, Derick)
* Added optional parameter to sha1(), sha1_file(), md5() and md5_file() which makes them return the digest as binary data. (Michael Bretterklieber, Derick)
* Added optional parameter to mkdir() to make directory creation recursive. (Ilia)
* Added optional parameter to file() which makes the result array not contain the line endings and to skip empty lines. (Ilia)
* Added new range() functionality:
o Support for float modifier. (Ilia)
o Detection of numeric values inside strings passed as high & low. (Ilia)
o Proper handle the situations where high == low. (Ilia)
o Added an optional step parameter. (Jon)
* Added encoding detection feature for expat XML parser. (Adam Dickmeiss, Moriyoshi)
* Added missing multibyte (unicode) support and numeric entity support to html_entity_decode(). (Moriyoshi)
* Added IPv6 support to ext/sockets. (Sara)
* Added input filter support. See README.input_filter for more info. (Rasmus)
* Added a replace count for str_[i]replace(), see #8218. (Sara)
* Fixed is_executable() to be available also on Windows. (Shane)
* Fixed dirname() and strip_tags() to be binary-safe. (Moriyoshi)
* Fixed bug #24098 (crash in pathinfo()). (Ilia)
* Fixed bug #21985 and #22064 (various mb_send_mail() issues). (Moriyoshi)
* Fixed bug #21600 (Assign by reference function call changes variable contents). (Zeev)
The full list of changes for PHP5 are here:
http://www.php.net/ChangeLog-5.php