1.6. Using PHP Variables
Variables are containers used to hold information. A variable has a name, and information is stored in the variable. For instance, you might name a variable $age and store the number 12 in it. After information is stored in a variable, it can be used later in the script. One of the most common uses for variables is to hold the information that a user types into a form.
1.6.1. Naming a variable
When you're naming a variable, keep the following rules in mind:
All variable names have a dollar sign ($) in front of them. This tells PHP that it is a variable name.
Variable names can be any length.
Variable names can include letters, numbers, and underscores only.
Variable names must begin with a letter or an underscore. They cannot begin with a number.
Uppercase and lowercase letters are not the same. For example, $firstname and $Firstname are not the same variable. If you store information in $firstname, for example, you can't access that information by using the variable name $firstName.