As shown in Figure 46-1 above, PHP can be extended primarily at
three points: external modules, built-in modules, and the Zend
engine. The following sections discuss these options.
External modules can be loaded at script runtime using the
function dl(). This function loads a shared
object from disk and makes its functionality available to the
script to which it's being bound. After the script is terminated,
the external module is discarded from memory. This method has both
advantages and disadvantages, as described in the following table:
External modules don't require recompiling of PHP.
The shared objects need to be loaded every time a script is
being executed (every hit), which is very slow.
The size of PHP remains small by "outsourcing" certain
External additional files clutter up the disk.
Every script that wants to use an external module's
functionality has to specifically include a call to
dl(), or the extension
tag in php.ini needs to be modified
(which is not always a suitable solution).
To sum up, external modules are great for
third-party products, small additions to PHP that are rarely used,
or just for testing purposes. To develop additional functionality
quickly, external modules provide the best results. For frequent
usage, larger implementations, and complex code, the disadvantages
outweigh the advantages.
Third parties might consider using the
extension tag in php.ini
to create additional external modules to PHP. These external
modules are completely detached from the main package, which is a
very handy feature in commercial environments. Commercial
distributors can simply ship disks or archives containing only
their additional modules, without the need to create fixed and
solid PHP binaries that don't allow other modules to be bound to
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.
Of course, extensions can also be implemented directly in the Zend
engine. This strategy is good if you need a change in the language
behavior or require special functions to be built directly into
the language core. In general, however, modifications to the Zend
engine should be avoided. Changes here result in incompatibilities
with the rest of the world, and hardly anyone will ever adapt to
specially patched Zend engines. Modifications can't be detached
from the main PHP sources and are overridden with the next update
using the "official" source repositories. Therefore, this method
is generally considered bad practice and, due to its rarity, is
not covered in this book.