I've been working with the Joomla framework and I have noticed that they use a convention to designate private or protected methods (they put an underscore "_" in front of the method name), but they do not explicitly declare any methods public, private, or protected. Why is this? Does it have to do with portability? Are the public, private, or protected keywords not available in older versions of PHP?
public, private and protected are PHP5 keywords. unfortunately, PHP4 still has a very high install base (especially amongst shared hosting services).
here's a pretty pic showing july usage rates (text in french). spoiler: php4 still has over a 35% usage rate sadly.
There's some good arguments for not using private methods and properties over here: http://aperiplus.sourceforge.net/visibility.php
Includes: makes coding and reading code harder. Makes re-using classes and debugging harder.
I'm not sold either way, but I would like to see an informed rebuttal of that article, if one exists.
PHP5 introduced some hefty changes in the object model. Among supporting visibility, there are various other changes. Be sure to check out:
来源：https://stackoverflow.com/questions/196737/why-not-use-protected-or-private-in-php