All keywords (e.g. if, else, while, echo, etc.), classes, functions, and user-defined functions are NOT case-sensitive, but all variable names are case-sensitive.
A variable starts with the $ sign, followed by the name of the variable
A variable name must start with a letter or the underscore character
A variable name cannot start with a number
A variable name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ )
Variable names are case-sensitive ($age and $AGE are two different variables)