Like other languages, PHP also have special keywords/reserved words, that have special meaning. The following words cannot be used as constants, class names, function or method names. Generally, It can be used as variable names, but could lead to confusion and should not be used.
Just a normal software developer who love to write code. Developing professional software since 1999. After working in the software industry for many years, I've started my own website to share knowledges and experiences.