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
No need to load the module specifically; the functionality is
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.