Understanding extensions in php.ini

I'm having trouble understanding these ;extension=xxx.dll files in php.ini

Is there any document I can refer to which explains these extensions in detai开发者_StackOverflow中文版l?

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The .dll files are for PHP extensions. You can read about all of them here: http://php.net/manual/en/install.windows.extensions.php

PHP extensions give you extra PHP functionality. They are basically function libraries that add features like MySQL functions, LDAP functions, and even Java functions.

To activate the extensions provided by the default PHP install, simply uncomment the ;extension=xxx.dll line (remove the semicolon) so that it just looks like extension=xxx.dll.

However, not all extensions are bundled with PHP. For example, the PECL extensions, including APC, have to be installed externally. Instructions for installing PECL can be found here: http://www.php.net/manual/en/install.pecl.php

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"Extension Writing Part I: Introduction to PHP and Zend"

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You only need to worry about those if you're running on a Windows server - since they appear to be commented out (; at the start of the line) I would assume you're on a Linux server?
In any case, you should be able to look up each extension individually on Google.