Rules for PHP variables:

• A variable starts with the $ sign
• 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 _ )

Scope:

• A variable declared within a function has a local scope and can only be accessed within that function
• Use the global keyword to reference a global variable

Constants:

• define("NAME", "value");
• Constants can be used anywhere