尽管大部分现有的 PHP 5 代码不需要更改就可以正常运行，但请注意一些不向后兼容的变更：
已放弃对 Windows XP 和 2003 的支持。构建 Windows 版本的 PHP 需要 Windows Vista 或更新的系统。
All case insensitive matching for function, class and constant names is now performed in a locale independent manner according to ASCII rules. This improves support for languages using the Latin alphabet with unusual collating rules, such as Turkish and Azeri.
This may cause issues for code that uses case insensitive matches for non-ASCII characters in multibyte character sets (including UTF-8), such as accented characters in many European languages. If you have a non-English, non-ASCII code base, then you will need to test that you are not inadvertently relying on this behaviour before deploying PHP 5.5 to production systems.
为使 pack() 和 unpack() 更兼容 Perl 做了一些变更：
Writing backward compatible code that uses the "a" format code with unpack() requires the use of version_compare(), due to the backward compatibility break.
例如：
<?php
// 以前的代码：
$data = unpack('a5', $packed);
// 新的代码：
if (version_compare(PHP_VERSION, '5.5.0-dev', '>=')) {
$data = unpack('Z5', $packed);
} else {
$data = unpack('a5', $packed);
}
?>
The GUIDs that previously resulted in PHP outputting various logos have been removed. This includes the removal of the functions to return those GUIDs. The removed functions are:
Extension authors should note that the zend_execute() function can no longer be overridden, and that numerous changes have been made to the execute_data struct and related function and method handling opcodes.
Most extension authors are unlikely to be affected, but those writing extensions that hook deeply into the Zend Engine should read the notes on these changes.