PHP Manual
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Built-in Modules

Built-in modules are compiled directly into PHP and carried around with every PHP process; their functionality is instantly available to every script that's being run. Like external modules, built-in modules have advantages and disadvantages, as described in the following table:

Advantages	Disadvantages
No need to load the module specifically; the functionality is instantly available.	Changes to built-in modules require recompiling of PHP.
No external files clutter up the disk; everything resides in the PHP binary.	The PHP binary grows and consumes more memory.

Built-in modules are best when you have a solid library of functions that remains relatively unchanged, requires better than poor-to-average performance, or is used frequently by many scripts on your site. The need to recompile PHP is quickly compensated by the benefit in speed and ease of use. However, built-in modules are not ideal when rapid development of small additions is required.