Predefined Variables in PHP: For complete beginners

This is an overview made for beginners: and introduction to predefined variables in PHP. We’ll go over what they are, where you’ve probably seen them, and how and when to use them.

What are predefined variables?

Predefined variables in PHP are exactly what they sound like — variables in the PHP language that have been defined already. They can use used in any PHP script, with no need to create them yourself. They serve a purpose that is commonly used among programmers repeatedly, such as passing information from a form to a PHP script ($_POST would be used for this).

Here is the list of currently defined variables in PHP. Of course, you don’t need to understand all of these quite yet, but they can come in handy for any PHP developer.

All of these variables interact with the current server in some way. This means their purpose is to either get information from the server (e.g. Find the IP address of the user) or translate information to the server (e.g. send a user’s information from a form through the server to be handled by a PHP script.)

Let’s now get a closer look at some of the most commonly used predefined variables.

$_SERVER

This variable is an array of values, with the values containing information about the server.

If you don’t know what an array is, it’s quite simple: it is a collection of values, put into one component. For example, an array of numbers would be “1, 4, 6, 3, 3″. All these numbers can be stored in one place, called an array. The array can then be assigned to a variable name, like “$numbers”. There are more formal definitions of an array, as this definition may not be 100% ‘accurate’. However, it’s a good outline and way to think of arrays.

Let’s say we’re creating a fluid layout, and because different browsers handle pixels differently, we want to determine the browser type of the user so we can lead them to a specified stylesheet. This way, we can make sure a fluid layout never breaks.

In the PHP $_SERVER array (collection), there is one piece called “HTTP_USER_AGENT”. This piece of the array (index of the array) contains the user’s browser information. We can call it like so:

$_SERVER["HTTP_USER_AGENT"];

To see it in action, let’s echo it out:

<?php
     echo $_SERVER["HTTP_USER_AGENT"];
?>

The code above will return browser information in the form of a string. My browser information looks like this:

Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.0.10) Gecko/2009042316 Firefox/3.0.10
     (.NET CLR 3.5.30729)

I won’t get too into it, but from here on we can see how we can use this information to specify a certain stylesheet for a user. If we store that String in a variable, (or part of the string), and send it through an if/else statement, we can give the user the correct stylesheet.

Note: This is just pseudo code below!

$browserType = $_SERVER["HTTP_USER_AGENT"];

if $browserType = "Mozilla..."
     then [switch user to the Mozilla Firefox stylesheet]

else if $browserType = "Internet Explorer..."
     then [switch user to the IE stylesheet]

else [use the default stylesheet]

Here are some of the other indexes in the $_SERVER array:

  • Get the user’s IP Address: REMOTE_ADDR
  • Get the hosting server’s name: SERVER_NAME
  • Retrieve the referrer URL: HTTP_REFERER
  • Retrieves the server identification string: SERVER_SOFTWARE
  • Retrieves the pathname of the current running script: SCRIPT_FILENAME
  • Retrieves the current page URL: SERVER_PROTOCOL
  • Retrieves how the script was requested (Did it use “Get” or “Post”): REQUEST_METHOD

If you don’t understand what they do based on the general description given above, do some more research on them, as well as try them out!

$_POST

This predefined variable is very commonly used, and incredibly useful when working with forms. You’ve probably seen this one before if you’ve ever tried to learn PHP.

The $_POST variable is linked to a method in PHP, called “post”, respectively. This method is often used in forms that need to send data to a PHP script. An example of this would be a sign up form: the user enters in data to the form, the form uses the post method to send it to the specified script, and the script can then process the data.

<form method="post" action="phpscript.php">

Easy enough, right? Well in order to carry the input from the form over to the script (in the example above, ‘phpscript.php’), it puts the data in an array, and assigns that array to a variable: $_POST. So, the $_POST variable contains all the data sent through the post method.

To specify the names of the indexes in the $_POST array, we clarify names in the input tags within the form:

<form method="post" action="phpscript.php">
<input name="username" type="text" />

After using the name=”" attribute, we can easily find that data in our $_POST variable array:

$_POST['username']

We have retrieved the data from a form, sent it to a PHP script, and can now work with it inside the PHP script. Using that, we can assign the username to a variable with in our PHP script, manipulate it, or send it to a database.

$_GET

The $_GET variable is very similar to the $_POST variable in that it can work with forms and a PHP method called “get”.

<form method="get" action="phpscript.php">

The difference is that the get method will send information through the URL, like so:

http://example.com/?name=Kayla&website=http://webitect.net

Let’s break that up a bit. We can see the example website: http://example.com. Then, we see a question mark (?). This question mark means that everything following it within this URL is now associated with the form, and contains the variables. We then see “name=Kayla”. This would be if I were to create an input field named “name”, and input “Kayla” for the variable to be passed:

<form method="get" action="thankyou.php">
<input name="name" type="text" />

Next, an ampersand (&) that will connect all the field names and variables. Finally, our second piece of data, named “website” with a value of “http://webitect.net”.

The URL will go to the page where the form wants us to go (the location of the action=”" attribute). In our case, ‘thankyou.php’. It can display a thank you message, or whatever other type of content you’d like to display. In addition, this page can retrieve data with PHP:

$_GET['name']

A better example of $_GET in action is:

<h1>Hello, <?php $_GET['name']; ?>!</h1>

Of course, that’s fairly useless, but you see how it could be used if needed. The point of $_GET is to easily pass information from a form that the user enters, and provide it to the next page. This is helpful for any sort of non-secure information, such as number of items to add to a shopping cart, or posting a website to a database.

You would not, however, ever want to use $_GET for a signup form or anything that uses secure information. Otherwise, important information (like a password) would be sent through the URL, and could be seen by anyone that could view the computer screen.

Conclusion

Hopefully with a more in-depth overview of these three predefined variables, it can get you started in understanding the rest of them. A great place to get started is the PHP Manual itself.

In a future tutorial I really want to go over the $_COOKIE variable, and how one can use cookies to identify users and work with specific users to the programmer’s advantage.

Written By Kayla

Kayla Knight is a 20 year old college student, part-time web developer, freelancer, and blogger. Webitect is where she spends too much of her freetime, sharing interesting finds and valuable resources. Be sure to check out her portfolio.

2 Comments

  1. Justin

    June 4th, 2009 at 11:16 am

    Wow…. I’m pretty fluent with php, but there were some unanswered questions about why things do what they do that you answered…. lol nice tutorial! very informative!

  2. Swetlana

    June 4th, 2009 at 12:55 pm

    Hey Kayla :)

    First off, wow! This site changed soooo much! And soooo much to the better! I really like you new and soo professionell style! Great job :)

    How is your competition going on? Any great entries?

    I don’t really hear anything on it.. which sucks. And my doctor has no idea what’s wrong. He says it’ll get better with the time. But I decided to make another appointment at a different doctor too, because I want to know what’s wrong with it.

    Life without a computer is soo difficult xD Especially if you’re used to it! I HAVE to check my site, talk to my online friends etc.

    Hope you’re doing great :) Bye <3