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Dealing with Forms

One of the most powerful features of PHP is the way it handles HTML forms. The basic concept that is important to understand is that any form element will automatically be available to your PHP scripts. Please read the manual section on Variables from external sources for more information and examples on using forms with PHP. Here is an example HTML form:

Example #1 A simple HTML form

There is nothing special about this form. It is a straight HTML form with no special tags of any kind. When the user fills in this form and hits the submit button, the action.php page is called. In this file you would write something like this:

Example #2 Printing data from our form

A sample output of this script may be:

Hi Joe. You are 22 years old.

Apart from the htmlspecialchars() and (int) parts, it should be obvious what this does. htmlspecialchars() makes sure any characters that are special in html are properly encoded so people can’t inject HTML tags or Javascript into your page. For the age field, since we know it is a number, we can just convert it to an int which will automatically get rid of any stray characters. You can also have PHP do this for you automatically by using the filter extension. The $_POST[‘name’] and $_POST[‘age’] variables are automatically set for you by PHP. Earlier we used the $_SERVER superglobal; above we just introduced the $_POST superglobal which contains all POST data. Notice how the method of our form is POST. If we used the method GET then our form information would live in the $_GET superglobal instead. You may also use the $_REQUEST superglobal, if you do not care about the source of your request data. It contains the merged information of GET, POST and COOKIE data.

You can also deal with XForms input in PHP, although you will find yourself comfortable with the well supported HTML forms for quite some time. While working with XForms is not for beginners, you might be interested in them. We also have a short introduction to handling data received from XForms in our features section.

User Contributed Notes 3 notes

According to the HTTP specification, you should use the POST method when you’re using the form to change the state of something on the server end. For example, if a page has a form to allow users to add their own comments, like this page here, the form should use POST. If you click «Reload» or «Refresh» on a page that you reached through a POST, it’s almost always an error — you shouldn’t be posting the same comment twice — which is why these pages aren’t bookmarked or cached.

You should use the GET method when your form is, well, getting something off the server and not actually changing anything. For example, the form for a search engine should use GET, since searching a Web site should not be changing anything that the client might care about, and bookmarking or caching the results of a search-engine query is just as useful as bookmarking or caching a static HTML page.

Also, don’t ever use GET method in a form that capture passwords and other things that are meant to be hidden.

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PHP Form Validation

This and the next chapters show how to use PHP to validate form data.

PHP Form Validation

Think SECURITY when processing PHP forms!

These pages will show how to process PHP forms with security in mind. Proper validation of form data is important to protect your form from hackers and spammers!

The HTML form we will be working at in these chapters, contains various input fields: required and optional text fields, radio buttons, and a submit button:

The validation rules for the form above are as follows:

Field	Validation Rules
Name	Required. + Must only contain letters and whitespace
E-mail	Required. + Must contain a valid email address (with @ and .)
Website	Optional. If present, it must contain a valid URL
Comment	Optional. Multi-line input field (textarea)
Gender	Required. Must select one

First we will look at the plain HTML code for the form:

Text Fields

The name, email, and website fields are text input elements, and the comment field is a textarea. The HTML code looks like this:

Radio Buttons

The gender fields are radio buttons and the HTML code looks like this:

The Form Element

The HTML code of the form looks like this:

When the form is submitted, the form data is sent with method=»post».

What is the $_SERVER[«PHP_SELF»] variable?

The $_SERVER[«PHP_SELF»] is a super global variable that returns the filename of the currently executing script.

So, the $_SERVER[«PHP_SELF»] sends the submitted form data to the page itself, instead of jumping to a different page. This way, the user will get error messages on the same page as the form.

What is the htmlspecialchars() function?

The htmlspecialchars() function converts special characters to HTML entities. This means that it will replace HTML characters like < and >with < and >. This prevents attackers from exploiting the code by injecting HTML or Javascript code (Cross-site Scripting attacks) in forms.

Big Note on PHP Form Security

The $_SERVER[«PHP_SELF»] variable can be used by hackers!

If PHP_SELF is used in your page then a user can enter a slash (/) and then some Cross Site Scripting (XSS) commands to execute.

Cross-site scripting (XSS) is a type of computer security vulnerability typically found in Web applications. XSS enables attackers to inject client-side script into Web pages viewed by other users.

Assume we have the following form in a page named «test_form.php»:

Now, if a user enters the normal URL in the address bar like «http://www.example.com/test_form.php», the above code will be translated to:

However, consider that a user enters the following URL in the address bar:

In this case, the above code will be translated to:

This code adds a script tag and an alert command. And when the page loads, the JavaScript code will be executed (the user will see an alert box). This is just a simple and harmless example how the PHP_SELF variable can be exploited.

Be aware of that any JavaScript code can be added inside the tag! A hacker can redirect the user to a file on another server, and that file can hold malicious code that can alter the global variables or submit the form to another address to save the user data, for example.

How To Avoid $_SERVER[«PHP_SELF»] Exploits?

$_SERVER[«PHP_SELF»] exploits can be avoided by using the htmlspecialchars() function.

The form code should look like this:

The htmlspecialchars() function converts special characters to HTML entities. Now if the user tries to exploit the PHP_SELF variable, it will result in the following output:

The exploit attempt fails, and no harm is done!

Validate Form Data With PHP

The first thing we will do is to pass all variables through PHP’s htmlspecialchars() function.

When we use the htmlspecialchars() function; then if a user tries to submit the following in a text field:

— this would not be executed, because it would be saved as HTML escaped code, like this:

The code is now safe to be displayed on a page or inside an e-mail.

We will also do two more things when the user submits the form:

1. Strip unnecessary characters (extra space, tab, newline) from the user input data (with the PHP trim() function)
2. Remove backslashes (\) from the user input data (with the PHP stripslashes() function)

The next step is to create a function that will do all the checking for us (which is much more convenient than writing the same code over and over again).

We will name the function test_input().

Now, we can check each $_POST variable with the test_input() function, and the script looks like this:

Example

// define variables and set to empty values
$name = $email = $gender = $comment = $website = «»;

if ($_SERVER[«REQUEST_METHOD»] == «POST») $name = test_input($_POST[«name»]);
$email = test_input($_POST[«email»]);
$website = test_input($_POST[«website»]);
$comment = test_input($_POST[«comment»]);
$gender = test_input($_POST[«gender»]);
>

function test_input($data) $data = trim($data);
$data = stripslashes($data);
$data = htmlspecialchars($data);
return $data;
>
?>

Notice that at the start of the script, we check whether the form has been submitted using $_SERVER[«REQUEST_METHOD»]. If the REQUEST_METHOD is POST, then the form has been submitted — and it should be validated. If it has not been submitted, skip the validation and display a blank form.

However, in the example above, all input fields are optional. The script works fine even if the user does not enter any data.

The next step is to make input fields required and create error messages if needed.

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PHP Form Handling

The PHP superglobals $_GET and $_POST are used to collect form-data.

PHP — A Simple HTML Form

The example below displays a simple HTML form with two input fields and a submit button:

Example

When the user fills out the form above and clicks the submit button, the form data is sent for processing to a PHP file named «welcome.php». The form data is sent with the HTTP POST method.

To display the submitted data you could simply echo all the variables. The «welcome.php» looks like this:

The output could be something like this:

The same result could also be achieved using the HTTP GET method:

Example

and «welcome_get.php» looks like this:

The code above is quite simple. However, the most important thing is missing. You need to validate form data to protect your script from malicious code.

Think SECURITY when processing PHP forms!

This page does not contain any form validation, it just shows how you can send and retrieve form data.

However, the next pages will show how to process PHP forms with security in mind! Proper validation of form data is important to protect your form from hackers and spammers!

GET vs. POST

Both GET and POST create an array (e.g. array( key1 => value1, key2 => value2, key3 => value3, . )). This array holds key/value pairs, where keys are the names of the form controls and values are the input data from the user.

Both GET and POST are treated as $_GET and $_POST. These are superglobals, which means that they are always accessible, regardless of scope — and you can access them from any function, class or file without having to do anything special.

$_GET is an array of variables passed to the current script via the URL parameters.

$_POST is an array of variables passed to the current script via the HTTP POST method.

When to use GET?

Information sent from a form with the GET method is visible to everyone (all variable names and values are displayed in the URL). GET also has limits on the amount of information to send. The limitation is about 2000 characters. However, because the variables are displayed in the URL, it is possible to bookmark the page. This can be useful in some cases.

GET may be used for sending non-sensitive data.

Note: GET should NEVER be used for sending passwords or other sensitive information!

When to use POST?

Information sent from a form with the POST method is invisible to others (all names/values are embedded within the body of the HTTP request) and has no limits on the amount of information to send.

Moreover POST supports advanced functionality such as support for multi-part binary input while uploading files to server.

However, because the variables are not displayed in the URL, it is not possible to bookmark the page.

Developers prefer POST for sending form data.

Next, lets see how we can process PHP forms the secure way!

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