Variables

You create variables to represent data. For instance, the following variable holds a value for sales tax:

 $sales_tax = 0.0875; 

This variable holds a SQL statement:

 $sql = "SELECT * FROM MY_TABLE"; 

You can refer to the value of other variables when determining the value of a new variable:

 $tax_total = $sales_tax * $sub_total; 

The following are true of variable names:

• They begin with a dollar sign ($).

• They cannot begin with a numeric character.

• They can contain numbers and the underscore character (_).

• They are case-sensitive.

Here are some common variable types:

• floats

• integers

• strings

These types are determined by PHP, based on the context in which they appear.

Floats

Each of the following variables is a float, or floating-point number. Floats are also known as "numbers with decimal points."

 $a = 1.552; $b = 0.964; $sales_tax = 0.875; 

Integers

Integers are positive or negative whole numbers, zero, or "numbers without decimal points." Each of the following variables is an integer.

 $a = 15; $b = -521; 

Strings

A series of characters grouped within double quotes is considered a string:

 $a - "I am a string."; $b = "<P>This book is <strong>cool</strong>!"; 

You can also reference other variables within your string, which will be replaced when your script is executed. For example:

 $num = 57; // an integer $my_string = "I read this book $num times!"; // a string 

When you run the script, $my_string will become "I read this book 57 times!"

Variables from HTML Forms

Depending on the method of your HTML form (GET or POST), the variables will be part of the $_POST or $_GET superglobal associative array. The name of the input field will become the name of the variable. For example, when a form is sent using the POST method, the following input field produces the variable $_POST[first_name]:

 <input type="text" name="first_name" size="20"> 

If the method of this form were GET, this variable would be $_GET[first_name].

Variables from Cookies

Like variables from forms, variables from cookies are kept in a superglobal associative array called $_COOKIE. If you set a cookie called user with a value of Joe Smith, like so:

 SetCookie ("user", "Joe Smith", time()+3600); 

a variable called user is placed in $_COOKIE, with a value of Joe Smith. You then refer to $_COOKIE[user] to get that value.

Environment Variables

When a Web browser makes a request of a Web server, it sends along with the request a list of extra variables called environment variables. They can be very useful for displaying dynamic content or authorizing users.

By default, environment variables are available to PHP scripts as $VAR_NAME. However, to be absolutely sure that you're reading the correct value, you can use the getenv() function to assign a value to a variable of your choice. Following are some common environment variables:

REMOTE_ADDR gets the IP address of the machine making the request. For example:

 <?php           $remote_address = getenv("REMOTE_ADDR");           echo "Your IP address is $remote_address."; ?> 

HTTP_USER_AGENT gets the browser type, browser version, language encoding, and platform. For example:

 <?php           $browser_type = getenv("HTTP_USER_AGENT");           echo "You are using $browser_type."; ?> 

For a list of HTTP environment variables and their descriptions, visit http://hoohoo.ncsa.uiuc.edu/cgi/env.html.

Arrays

Simply put, arrays are sets of variables that are contained as a group. In the following example, $fave_colors is an array that contains strings representing array elements. In this case, the array elements (0 to 3) are names of colors.

 $fave_colors[0] = "red"; $fave_colors[1] = "blue"; $fave_colors[2] = "black"; $fave_colors[3] = "white"; 

Array elements are counted with 0 as the first position in the numerical index.