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:
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.