Variables in php

PHP Programming Language

Remember this about variables:

1. A variable starts with the $ sign, followed by the name of the variable
2. A variable name must start with a letter or the underscore character
3. A variable name cannot start with a number
4. A variable name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ )
5. Variable names are case-sensitive ($age and $AGE are two different variables)

A unit of abstract mathematical system subject to the laws of arithmetic.