PHP Form Validation
PHP Form Validation
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:
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>
PHP Form Validation Example
* required field.Your Input:
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:
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
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".
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.
How To Avoid $_SERVER["PHP_SELF"] Exploits?
$_SERVER["PHP_SELF"] exploits can be avoided by using the htmlspecialchars() function.
The form code should look like this:
<form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>">
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:
<form method="post" action="test_form.php/"><script>alert('hacked')</script>">
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:
<script>location.href('http://www.hacked.com')</script>
- this would not be executed, because it would be saved as HTML escaped code, like this:
<script>location.href('http://www.hacked.com')</script>
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:
<?php
// 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.
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