The following characters are reserved and cannot be used as identifiers in your programs. They have already been used by the Java language itself.
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.
The following keywords are reserved. They cannot be used as variables, functions, methods, or object identifiers. You can bypass this restriction by enclosing the name in brackets [ ] but not recommended.
Many Programming languages have predefined keywords that have special meanings to the compiler. They cannot be used as variables, functions, methods, or object identifiers. The following words are reserved for C# language itself.