A Practical Guide to PHP Form Validation By Examples

Summary: in this tutorial, you’ll learn about PHP form validation, how to validate form data, and how to show error messages if the user inputs are invalid.

Introduction to PHP form validation

When processing a form, it’s critical to validate user inputs to ensure that the data is in a valid format.

There are two types of validations: client-side & server-side:

The client-side validation is performed in the web browsers of the users. To validate data at the client side, you can use HTML5 validation or JavaScript. The client-side validation aims to assist legitimate users in entering data in the valid format before submitting it to the server.

However, client-side validation doesn’t prevent malicious users from submitting data that can potentially exploit the application.

The server-side validation validates data in the web server using PHP. To validate data in PHP, you can use the filter_var() and filter_input() functions.

PHP form validation example

We’ll build an email subscription form that includes a validation feature. The form has the name and email input elements and a submit button:

If you don’t enter the name and/or email and click the subscribe button, the form will show the error messages. Also, if you enter an invalid email address, the form will show a different error message.

Notice that we don’t use the client-side validation for this form to make it easier to test. In practice, you should also use client-side validation.

Organize the directory and files

First, create a file and directory structure as follows:

.
├── css
│   └── style.css
├── inc
│   ├── get.php
│   ├── post.php
│   ├── header.php
│   ├── footer.php
│   └── .htaccess
└── index.phpCode language: plaintext (plaintext)

The following table describes the purpose of each file:

File	Directory	Description
index.php	.	Contain the main logic of the form
header.php	inc	Contain the header code
footer.php	inc	Contain the footer code
get.php	inc	Contain the email subscription form
post.php	inc	Contain the code for handling form submission
style.css	css	Contain the CSS code

header.php

The following shows the header.php file:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link rel="stylesheet" href="css/style.css">
    <title>Subscribe</title>
</head>
<body>
    <main>Code language: HTML, XML (xml)

The header.php file link to the style.css file in the css directory.

footer.php

And the footer.php only contains the enclosing tags that correspond to the opening tags in the header.php file:

</main>
</body>
</html>Code language: HTML, XML (xml)

index.php

The index.php file contains the main logic of the form:

<?php

require __DIR__ . '/inc/header.php';

$errors = [];
$inputs = [];

$request_method = strtoupper($_SERVER['REQUEST_METHOD']);

if ($request_method === 'GET') {
    // show the form
    require __DIR__ . '/inc/get.php';
} elseif ($request_method === 'POST') {
    // handle the form submission
    require    __DIR__ .  '/inc/post.php';
    // show the form if the error exists
    if (count($errors) > 0) {
        require __DIR__ . '/inc/get.php';
    }
}

require __DIR__ .  '/inc/footer.php';
Code language: HTML, XML (xml)

How the index.php works.

First, load code from both header.php and footer.php files using the require construct at the top and bottom of the file to generate the header and footer.

Second, define the $errors array to store error messages and the $inputs array to store the entered form values. If an input element has invalid data, the index.php will show the entered value stored in the $inputs.

Third, show the form if the HTTP request method is GET by loading the get.php file. Once you enter the

Finally, load the code in the post.php to handle the form submission if the HTTP request method is POST. If the form has any errors, the $errors will not empty. In this case, show the form again with the error messages stored in the $errors array and entered values stored in the $inputs arrays.

get.php

The get.php file contains the form:

<form action="<?php echo htmlspecialchars($_SERVER['PHP_SELF']) ?>" method="post">
    <header>
        <h1>Get FREE Updates</h1>
        <p>Join us for FREE to get email updates!</p>
    </header>
    <div>
        <label for="name">Name:</label>
        <input type="text" name="name" id="name" placeholder="Full Name" value="<?php echo $inputs['name'] ?? '' ?>" class="<?php echo isset($errors['name']) ? 'error' : ''  ?>">
        <small><?php echo $errors['name'] ?? '' ?></small>
    </div>
    <div>
        <label for="name">Email:</label>
        <input type="text" name="email" id="email" placeholder="Email Address" value="<?php echo $inputs['email'] ?? '' ?>" class="<?php echo isset($errors['email']) ? 'error' : '' ?>">
        <small><?php echo $errors['email'] ?? '' ?></small>
    </div>
    <button type="submit">Subscribe</button>
</form>Code language: HTML, XML (xml)

How the get.php works.

First, fill the name and email input elements with the entered values stored in the $inputs array only if these values exist.
Second, show the error messages stored in the $errors array if they exists.

post.php

The following shows the code of the post.php file. The post.php validates the form data using the filter_input() and filter_var() functions.

<?php

const NAME_REQUIRED = 'Please enter your name';
const EMAIL_REQUIRED = 'Please enter your email';
const EMAIL_INVALID = 'Please enter a valid email';

// sanitize and validate name
$name = filter_input(INPUT_POST, 'name', FILTER_SANITIZE_STRING);
$inputs['name'] = $name;

if ($name) {
    $name = trim($name);
    if ($name === '') {
        $errors['name'] = NAME_REQUIRED;
    }
} else {
    $errors['name'] = NAME_REQUIRED;
}


// sanitize & validate email
$email = filter_input(INPUT_POST, 'email', FILTER_SANITIZE_EMAIL);
$inputs['email'] = $email;
if ($email) {
    // validate email
    $email = filter_var($email, FILTER_VALIDATE_EMAIL);
    if ($email === false) {
        $errors['email'] = EMAIL_INVALID;
    }
} else {
    $errors['email'] = EMAIL_REQUIRED;
}
?>

<?php if (count($errors) === 0) : ?>
    <section>
        <h2>
            Thanks <?php echo htmlspecialchars($name) ?> for your subscription!
        </h2>
        <p>Please follow the steps below to complete your subscription:</p>
        <ol>
            <li>Check your email (<?php echo htmlspecialchars($email) ?>) - Find the message sent from [email protected]</li>
            <li>Click to confirm - Click on the link in the email to confirm your subscription.</li>
        </ol>
    </section>

<?php endif ?>Code language: HTML, XML (xml)

How it works.

First, define some constants to store the error messages. In a real-world application, you can store all the messages in a separate file:

const NAME_REQUIRED = 'Please enter your name';
const EMAIL_REQUIRED = 'Please enter your email';
const EMAIL_INVALID = 'Please enter a valid email';
Code language: JavaScript (javascript)

Second, sanitize and validate the name using the filter_input() function. If the name is empty, add an error message to the $errors array.

// sanitize and validate name
$name = filter_input(INPUT_POST, 'name', FILTER_SANITIZE_STRING);
$inputs['name'] = $name;

if ($name) {
    $name = trim($name);
    if ($name === '') {
        $errors['name'] = NAME_REQUIRED;
    }
} else {
    $errors['name'] = NAME_REQUIRED;
}Code language: PHP (php)

Third, sanitize and validate email using the filter_input() and filter_var() functions. If the email is empty or invalid, add the corresponding error message to the $errors array.

// sanitize & validate email
$email = filter_input(INPUT_POST, 'email', FILTER_SANITIZE_EMAIL);
$inputs['email'] = $email;
if ($email) {
    // validate email
    $email = filter_var($email, FILTER_VALIDATE_EMAIL);
    if ($email === false) {
        $errors['email'] = EMAIL_INVALID;
    }
} else {
    $errors['email'] = EMAIL_REQUIRED;
}Code language: PHP (php)

Finally, if the form has no error, show the confirmation message:

<?php if (count($errors) === 0) : ?>
    <section>
        <h2>
            Thanks <?php echo htmlspecialchars($name) ?> for your subscription!
        </h2>
        <p>Please follow the steps below to complete your subscription:</p>
        <ol>
            <li>Check your email (<?php echo htmlspecialchars($email) ?>) - Find the message sent from [email protected]</li>
            <li>Click to confirm - Click on the link in the email to confirm your subscription.</li>
        </ol>
    </section>
<?php endif ?>Code language: HTML, XML (xml)

To complete the form, you can save the contact data to a database or call an API of an email marketing service to add the contact to your list.

Summary

Use filter_input() and filter_var() functions to validate and sanitize data.

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