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Migrating from PHP 5.1.x to PHP 5.2.x
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## New INI Configuration Directives

New php.ini directives introduced in PHP 5.2.0:

- allow_url_include This useful option makes it possible to differentiate between standard file operations on remote files, and the inclusion of remote files. While the former is usually desirable, the latter can be a security risk if used naively. Starting with PHP 5.2.0, you can allow remote file operations while disallowing the inclusion of remote files in local scripts. In fact, this is the default configuration.
- pcre.backtrack_limit PCRE's backtracking limit.
- pcre.recursion_limit PCRE's recursion limit. Please note that if you set this value to a high number you may consume all the available process stack and eventually crash PHP (due to reaching the stack size limit imposed by the Operating System).
- session.cookie_httponly Marks the cookie as accessible only through the HTTP protocol. This means that the cookie won't be accessible by scripting languages, such as JavaScript. This setting can effectively help to reduce identity theft through XSS attacks (although it is not supported by all browsers).

New directives in PHP 5.2.2:

- max_input_nesting_level Limits how deep input variables can be nested, default is 64.

New Class Constants
Error Reporting
Migrating from PHP 5.1.x to PHP 5.2.x
PHP Manual