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:
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.