Identifiers give names to the abstract parts of PHP: functions, variables, and classes. Some of them are created by PHP in the form of built-in functions or environment variables. Others you create. Identifiers may be of any length and can consists of letters, numbers, or underscores. The first character of an identifier must be either a letter or an underscore. Table 2.1 contrasts acceptable identifiers with unacceptable ones.
Upper- and lowercase letters are recognized as different. That is, the variables UserName and username are two distinct identifiers. The exception is built-in functions. As stated in Chapter 1, "An Introduction to PHP," functions like print can be called as Print if you prefer.