Learn PHP Form Validation

PHP Form Validation

PHP has many built-in functions to validate different 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:

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:

                Name: <input type="text" name="name">
                E-mail: <input type="text" name="email">
                Website: <input type="text" name="website">
                Comment: <textarea name="comment" rows="5" cols="40"></textarea>

Radio Buttons

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

                Gender:
                <input type="radio" name="gender" value="female">Female
                <input type="radio" name="gender" value="male">Male
                <input type="radio" name="gender" value="other">Other

The Form Element

The HTML code of the form looks like this:

<form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>">

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 into 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.

Warning!

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":

<form method="post" action="<?php echo $_SERVER["PHP_SELF"];?>">

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:

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

So far, so good.
However, consider that a user enters the following URL in the address bar:

http://www.example.com/test_form.php/%22%3E%3Cscript%3Ealert('hacked')%3C/script%3E

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

<form method="post" action="test_form.php/"><script>alert('hacked')</script>

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:

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:
Strip unnecessary characters (extra space, tab, newline) from the user input data (with the PHP trim() function)
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.