Variables in PHP are quite different from compiled languages such as C and Java. This is because their weakly typed nature, which in short means you don’t need to declare variables before using them, you don’t need to declare their type and, as a result, a variable can change the type of its value as much as you want.

Variables in PHP are preceded with a $ sign, and similar to most modern languages, they can start with a letter (A-Za-z) or _ (underscore) and can then contain as many alphanumeric characters and underscores as you like.

Examples of legal variable names include

$count

$_Obj

$A123

Example of illegal variable names include

$123

$*ABC

As previously mentioned, you don’t need to declare variables or their type before using them in PHP. The following code example uses variables:

$PI = 3.14;

$radius = 5;

$circumference = $PI * 2 * $radius; // Circumference = ð * d

You can see that none of the variables are declared before they are used. Also, the fact that $PI is a floating-point number, and $radius (an integer) is not declared before they are initialized.

PHP does not support global variables like many other programming languages (except for some special pre-defined variables). Variables are local to their scope, and if created in a function, they are only available for the lifetime of the function. Variables that are created in the main script (not within a function) aren’t global variables; you cannot see them inside functions, but you can access them by using a special array $GLOBALS[], using the variable’s name as the string offset. The previous example can be rewritten the following way:

$PI = 3.14;

$radius = 5;

$circumference = $GLOBALS["PI"] * 2 * $GLOBALS["radius"]; // Circumference = ð * d

You might have realized that even though all this code is in the main scope (we didn’t make use of functions), you are still free to use $GLOBALS[], although in this case, it gives you no advantage.

2008-11-21

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