As shown in 圖形 24-1 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:
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 them.