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Core Extensions
These are not actual extensions. They are part of the PHP core and cannot be left out of a PHP binary with compilation options.
Bundled Extensions
These extensions are bundled with PHP.
External Extensions
These extensions are bundled with PHP but in order to compile them, external libraries will be needed.
PECL Extensions
These extensions are available from » PECL. They may require external libraries. More PECL extensions exist but they are not documented in the PHP manual yet.