PHP is a web template system that accidentally grew up into a fairly general language. PHP's syntax, capabilities, and execution model bear vague similarities to Perl; scripts are loaded by an "interpreter", compiled to bytecode, and then executed. The PHP interpreter can be run from the command line, CGI-style, or more commonly as an in-process Apache module.
Please see http://www.php.net/manual/en/installation.php.
Compile-time options 
The following extensions are required by MediaWiki:
- Perl Compatible Regular Expressions
- Standard PHP Library
- At least one database driver:
Perl Compatible Regular Expressions, Session and Standard PHP Library are enabled by default; Perl Compatible Regular Expressions and Standard PHP Library cannot be disabled since PHP 5.3.
MediaWiki either needs or wants a number of optional features of PHP that need to be enabled at compile time:
- mbstring multibyte character string support (optional; slower custom code will be used if not available)
- iconv character set conversion library (optional; other conversion functions will be used if not available)
- zlib compression library, optionally to compress the file cache
- sockets support for network communication, if using memcached
- dba DBM-style database abstraction layer, for caching, with the following handlers:
We are successfully working with these compiler options:
'./configure' \ '--with-mysql' \ '--with-apxs2=/usr/sbin/apxs2' \ '--with-zlib' \ '--with-ldap' \ '--with-gd' \ '--with-jpeg-dir' \ '--with-iconv-dir' \ '--enable-mbstring'
In detail, these configure options do the following:
- Allows PHP to query MySQL. This is an essential feature (I think). You will need to have MySQL installed to enable this option.
- this is required if you want to build mod_php. APXS is for configuring compilation of an Apache module. You will need to change (or omit the path) for your specific system.
- Allows PHP to read / write zipped files and send compressed data to Apache ... or something like that
- Important if you want your wiki users to be verified by LDAP
- Use this if you want image processing 'within' PHP, else you can use ImageMagick. Because this option requires that you have various libraries installed... but presumably IM has the same deps... oh well...
- Seems nonsensical
- Seems nonsensical
- Allows multibyte character string support. This is optional, as slower custom code will be used if not available.
Opcode caching 
- see Manual:Cache
Due to MediaWiki's large codebase, it is highly recommended that a system be used to cache the compiled scripts; there are a number of such plugins for PHP, some free, some proprietary. There is a list of some caches:
- Alternative PHP Cache -- used at Wikimedia
- Windows Cache Extension for PHP -- recommended for IIS users
Fortunately, these all seem to be easy to install as Zend plugins, you just drop in a library and change your php.ini. No fussy recompiling of the entire PHP!
Depending on the cache and options used, you may have to perform a special operation whenever updating script files.
MediaWiki can also interact with that caches and use them for object caching, see Manual:$wgMainCacheType.
Runtime configuration (php.ini) 
Some settings in php.ini affects MediaWiki behaviour and some of them are incompatible with MediaWiki
- Warning: Enabling this is a major security risk and can be used for XSS attacks; MediaWiki doesn't require it, so please turn it off if you can!
- Safe mode is an ill-conceived, broken-by-design setting in PHP; MediaWiki can work with this option on, but some features won't work.
- This options adds quotes to data in
$_COOKIES. MediaWiki can work with this option turned on, but will have to remove these quotes and thus will be slower.
- This option corrupts data input unpredictably by adding quotes in e.g. opened files, data received from database etc. MediaWiki will refuse to install if this option is turned on.
- same as magic_quotes_runtime
- This option causes errors and may corrupt data unpredictably; MediaWiki will refuse to install if this option is turned on.
- This option causes horrible bugs; MediaWiki will refuse to install if this option is turned on.
- MediaWiki needs sufficiently memory to work. The minimal limit should be something like 20 Mo, but if you want MediaWiki to work correctly, consider using at least 50 Mo.
Image Resizing 
ImageMagick can be used for image resizing When it is not available the GD PHP module is used instead where found.
See also 
- PHP Knowledgebase
- PHP Tutorials
- Suhosin (Hardened-PHP Project patch and extension)
- Simple PHP Tutorials
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