What are Variables ?

A variable is best thought of as a container for data. Once data has been stored in a variable (or, put differently, once a variable has been assigned a value), that data/variable can be altered , printed to the Web browser (when I say printed, it may help to think of it as sent, but it's the print statement that does the sending, so either term is appropriate), saved to a database, e-mailed, and so forth.

Variables are, by their nature, flexible: you can put data into a variable, retrieve that data from it (without affecting the value of the variable itself), put new data in, and you can continue this cycle as long as is necessary. But, variables in PHP are also temporary: they only exist ”that is, they only have a value ”while they are used within a script. Once you are in a new page, those variables cease to exist, unless you pass them along to the new page, which I'll discuss in the next chapter (HTML Forms and PHP).

Variable Syntax

In PHP, all variables begin with a dollar sign ($), followed by the variable name itself. This name must begin with either a letter (A “Z, a “z) or the underscore (_), followed by any number of letters , underscores, or numbers , used in combination or not. You may not use spaces within the name of a variable. Instead, the underscore is commonly used to separate words in a variable name.

Keep in mind that variables are case-sensitive. Consequently, "$variable" and "$Variable" are two different constructs, although it would never make sense to use two variables with such similar names. One should quickly get into the habit of creating variable names that make sense on their own, as well as using comments to indicate the purpose of variables (Script 2.1). These habits will reduce errors and make revisiting your work less taxing. For example, "$FirstName" is more useful than "$FN" and putting in a comment that details what a variable's purpose is will make your work abundantly clear. In fact, you may decide that "$first_name" is a better variable name than "$FirstName" because there are no capital letters to get right and the words are separated for clarity. No matter how you decide to name your variables, the most important thing to remember is that whatever convention you use, be consistent. This will help you avoid making trivial errors in your programming.

Unlike some other languages, in PHP you neither have to declare what a variable is (to declare a variable is to assign it a type ”I'll cover variable types in Types of Variables ) nor initialize it prior to first use (to initialize a variable is to create it). With PHP, a variable exists and is defined the first time you use it.