Beginner-Friendly PHP Input Validation Filtering Functions Usage Examples

Credits: pexels.com (Photo by Shvetsa)

You can spot a rookie PHP project from a mile away, inputs just scooped up and dropped into file includes, no questions asked. It’s the kind of thing we see all the time at our secure development bootcamp, and, honestly, it’s how trouble starts.

Remote File Inclusion (RFI) and Local File Inclusion (LFI) attacks love this sort of thing, letting attackers run their own code or poke around in places they shouldn’t. We’ve learned the hard way that using PHP’s input validation and filtering functions isn’t just a nice-to-have, it’s a must.

These tools help us scrub, check, and control everything coming into our apps, cutting off a bunch of common attacks at the knees. Here’s how we use them, with examples pulled straight from the trenches.

Key Takeaway

• PHP filtering functions like filter_var() and filter_input() are our go-to for keeping user inputs in check.
• Good input validation is the difference between a safe app and one that’s wide open to RFI and LFI.
• We always combine filtering with whitelists, absolute paths, and tight server settings, no single trick is enough on its own.

Understanding PHP Input Validation and Filtering Functions

Importance of Input Validation in PHP

Preventing Security Vulnerabilities like RFI and LFI

Every time our PHP apps take something from a user, there’s a chance it’s not what we expect. RFI and LFI are classic examples, attackers slip in a URL or file path, and if we’re not careful, the server just goes along with it.

That’s how you end up with remote code running on your box or private data leaking out. Input validation is our first line of defense, making sure only what we expect gets through.

Ensuring Data Integrity and Application Stability

It’s not just about security, either. We’ve seen apps crash or act weird because someone put “banana” in an age field. When we use filtering functions, the data stays clean, and the app stays up. Simple as that.

Common User Input Sources in PHP Applications

GET, POST, COOKIE, and FILES Arrays

Most of our user data comes in through superglobals, $_GET, $_POST, $_COOKIE, and $_FILES. These are all wide open unless we lock them down. Attackers know how to mess with every one of them, so we treat them all as suspects.

External APIs and User-Generated Content

It’s not just forms and cookies. Sometimes we pull in stuff from APIs or let users upload files or post comments. We’ve learned to always run these through our filters, too. If it’s coming from outside, it gets checked.

Overview of PHP Filtering Functions

filter_var() and filter_input()

PHP hands us filter_var() for checking or cleaning a single value, and filter_input() for grabbing and filtering straight from the superglobals. For example, filter_var($email, FILTER_VALIDATE_EMAIL) tells us if we’ve actually got an email address. There are built-in filters for all the usual suspects. (1)

filter_var_array() for Multiple Inputs

When we’re dealing with a whole form, filter_var_array() lets us set up rules for each field and run them all at once. Saves time and keeps things tidy.

Practical Usage of PHP Filtering Functions for Secure Input Handling

Credits: Traversy Media

Using filter_var() with Built-in Filters

Validating Email, URL, and Integer Inputs

We run into this every day, checking emails, URLs, or numbers. Here’s what it looks like:

$email = “user@example.com”;

if (filter_var($email, FILTER_VALIDATE_EMAIL)) {

    echo “Valid email”;

} else {

    echo “Invalid email”;

}

$url = “https://example.com”;

if (filter_var($url, FILTER_VALIDATE_URL)) {

    echo “Valid URL”;

}

$age = “25”;

if (filter_var($age, FILTER_VALIDATE_INT, [“options” => [“min_range” => 18, “max_range” => 99]])) {

    echo “Valid age”;

}

This way, we know the data fits what we expect.

Sanitizing Strings to Remove Dangerous Characters

Sometimes we just want to clean things up, not just check. For example, stripping out HTML tags:

$dirty_string = “<script>alert(‘xss’);</script>”;

$clean_string = filter_var($dirty_string, FILTER_SANITIZE_STRING);

echo $clean_string; // Outputs alert(‘xss’);

It’s not bulletproof for XSS, but it knocks out the obvious stuff.

Applying filter_input() for Superglobals

Filtering GET and POST Data Securely

We never grab data straight from $_GET or $_POST anymore. Instead, we use filter_input():

$username = filter_input(INPUT_POST, ‘username’, FILTER_SANITIZE_STRING);

$page = filter_input(INPUT_GET, ‘page’, FILTER_SANITIZE_STRING);

This keeps us from forgetting to clean things up.

Combining Validation and Sanitization in One Step

We usually sanitize first, then validate:

$email = filter_input(INPUT_POST, ’email’, FILTER_SANITIZE_EMAIL);

if (filter_var($email, FILTER_VALIDATE_EMAIL)) {

    echo “Email is valid and sanitized”;

}

Handling Multiple Inputs with filter_var_array()

Defining Filters for Each Input Field

For bigger forms, we set up filters for each field:

$filters = [

    ’email’ => FILTER_VALIDATE_EMAIL,

    ‘age’ => [

        ‘filter’ => FILTER_VALIDATE_INT,

        ‘options’ => [‘min_range’ => 18, ‘max_range’ => 99]

    ],

    ‘website’ => FILTER_VALIDATE_URL,

];

$inputs = filter_var_array($_POST, $filters);

We get back an array with each value checked or set to null if it’s not right.

Example: Validating a User Registration Form

A registration form usually has a few fields, username, email, age. Here’s how we handle it:

$filters = [

    ‘username’ => FILTER_SANITIZE_STRING,

    ’email’ => FILTER_VALIDATE_EMAIL,

    ‘age’ => [

        ‘filter’ => FILTER_VALIDATE_INT,

        ‘options’ => [‘min_range’ => 13, ‘max_range’ => 120]

    ],

];

$userData = filter_var_array($_POST, $filters);

if ($userData[’email’] && $userData[‘age’]) {

    // Proceed with registration

} else {

    // Handle invalid input

}

It keeps things organized and easy to follow.

Custom Callback Filters for Complex Validation Needs

Creating User-Defined Functions for Specific Scenarios

Sometimes the built-in stuff doesn’t cut it. Like when we want usernames to match a certain pattern:

function validate_username($username) {

    return preg_match(‘/^[a-zA-Z0-9_]{5,20}$/’, $username) ? $username : false;

}

$username = filter_var($input_username, FILTER_CALLBACK, [‘options’ => ‘validate_username’]);

if ($username !== false) {

    echo “Username valid”;

}

Callbacks give us room to handle weird cases.

Integrating with filter_var() for Enhanced Security

Mixing callbacks with filter_var() lets us keep our code clean but flexible.

Preventing File Inclusion Vulnerabilities with Input Validation

Risks of Remote File Inclusion (RFI) and Local File Inclusion (LFI)

We’ve seen what happens when someone includes a file based on unsanitized input, attackers can pull in remote scripts or peek at sensitive files. It’s a mess.

How Unsanitized Inputs Lead to Arbitrary File Execution

If someone can pass in http://evil.com/malicious.php or ../../../../etc/passwd, and the app just runs with it, you’re in trouble. That’s why we always filter before including.

Common Attack Techniques: Directory Traversal and Remote URLs

Attackers love using ../ to climb up directories or http:// to pull in their own code. We watch for these patterns and block them. (2)

Validating File Paths Using Filtering Functions

Removing or Rejecting Suspicious Characters (../, http://)

We usually reject anything with ../ or a URL scheme:

$page = $_GET[‘page’];

if (strpos($page, ‘../’) !== false || preg_match(‘/^https?:\/\//’, $page)) {

    die(‘Invalid file path’);

}

Using basename() and Realpath() for Path Normalization

basename() and realpath() help us keep includes inside the right folder:

$page = basename($_GET[‘page’]);

$fullPath = realpath(__DIR__ . ‘/pages/’ . $page);

if ($fullPath && strpos($fullPath, __DIR__ . ‘/pages/’) === 0) {

    include $fullPath;

} else {

    echo “Invalid file”;

}

That way, nothing gets included from outside our intended directory.

Implementing Whitelists with Filtering

Defining Allowed Filenames or Paths

We like to set up a whitelist of allowed files:

$whitelist = [‘home.php’, ‘about.php’, ‘contact.php’];

$page = $_GET[‘page’];

if (in_array($page, $whitelist)) {

    include __DIR__ . ‘/pages/’ . $page;

} else {

    include __DIR__ . ‘/pages/404.php’;

}

No surprises this way.

Combining filter_var() with in_array() Checks

We’ll sanitize first, then check against the whitelist:

$page = filter_var($_GET[‘page’], FILTER_SANITIZE_STRING);

if (in_array($page, $whitelist)) {

    include __DIR__ . ‘/pages/’ . $page;

}

Restricting Input Types and Lengths

We set limits on what a filename can look like and how long it can be:

$page = filter_var($_GET[‘page’], FILTER_VALIDATE_REGEXP, [

    ‘options’ => [‘regexp’ => ‘/^[a-zA-Z0-9_-]{1,20}\.php$/’]

]);

Rejects anything sketchy.

Limiting Input Size to Prevent Buffer Overflows

Even though PHP handles strings pretty well, we don’t let things get out of hand:

if (strlen($page) > 50) {

    die(‘Input too long’);

}

Best Practices for Secure PHP Input Handling and File Inclusion

Credits: pexels.com (Photo by Shvetsa)

Disabling Dangerous PHP Settings

We always turn off allow_url_include and allow_url_fopen in php.ini:

allow_url_include = Off

allow_url_fopen = Off

This shuts down remote files included at the server level.

Using Absolute Paths and Constants for Includes

Relative paths are risky. We use constants:

define(‘BASE_PATH’, __DIR__ . ‘/pages/’);

include BASE_PATH . ‘home.php’;

Keeps our includes where we want them.

Avoiding User-Controlled Relative Paths

We never let users decide the path on their own. Everything gets checked.

Combining Input Filtering with Access Controls

Even if someone finds a way to try LFI, we make sure sensitive files are locked down with permissions.

Employing Web Application Firewalls (WAF) for Additional Protection

A WAF can spot things like ../ or http:// in requests and block them before they even hit our app.

Error Handling and Logging Strategies

We don’t show users detailed errors. If something breaks, we log it quietly for ourselves.

Regular Updates and Security Audits

We keep PHP and all our libraries up to date. Every so often, we go back and review our code, especially the parts that deal with input.

Conclusion

We’ve found that input validation and filtering in PHP are non-negotiable if you want to avoid headaches like RFI and LFI. Functions like filter_var(), filter_input(), and filter_var_array() are our daily tools for keeping data clean. Whitelists, absolute paths, server settings, and good file permissions all work together to keep our apps safe. 

Want to get hands-on with secure coding skills that actually stick? Join the Secure Coding Practices Bootcamp, practical, expert-led, and designed for developers who ship code. 

FAQ

How does php input validation work with php filter_input usage and php input filtering?

php input validation works best when paired with php input filtering and proper php filter_input usage. These help keep your app from accepting bad or dangerous data.

You can pull user data using php filter input post, php filter input get, or php filter input request, then clean it using php sanitize input or validate it with things like php validate int or php validate email. Combining all of these helps keep data clean and secure from the start.

What are common php filter_var examples that show safe php input validation?

Some php filter_var examples include checking email using php validate email, URLs with php validate url, or integers with php validate int. You can also clean input using php filter sanitize string or php filter sanitize email.

These functions make it easier to apply php input validation and php input sanitization best practices in everyday code. Use them together to guard your app against invalid or risky input.

How do I use php filter array validation and php filter multiple inputs in forms?

When handling multiple fields, php filter array validation lets you apply filters to each one. You can combine it with php filter multiple inputs and define each rule in a php input filter options array.

Use php input filter required fields for must-have data and php input filter optional fields where input isn’t always needed. For each, use php validate float, php validate boolean, or php filter sanitize special chars depending on what you’re checking.

Can php input filtering prevent attacks like cross-site scripting or SQL injection?

Yes, php input filtering is key to stopping threats like php input filter cross-site scripting or php input filter sql injection prevention. It helps by removing risky data using php input filter strip tags or php input filter encode quotes.

To clean text fields, try php filter sanitize string and for safety, always follow php input filter xss prevention and php input filter secure coding practices. It’s a solid way to cut out bad input early.

What role does php input filter security play in file inclusion and remote code execution risks?

php input filter security helps prevent serious risks like php input filter file inclusion prevention or php input filter remote code execution prevention. Use filters like php filter validate regex for patterns and php input filter data type checking to ensure data behaves as expected.

Combine this with php input filter data range checking, php input filter data format checking, and php input filter error handling to catch mistakes before they lead to security issues.

How can I clean data using php filter sanitize float and php filter sanitize int?

Use php filter sanitize float to clean numbers with decimals, and php filter sanitize int for whole numbers. These help enforce php input filter data consistency and php input filter data type checking.

If your input has symbols or letters mixed in, these filters strip them out. Use them alongside php input filter trim or php input filter strip low to clean further, then validate with php validate float range or php validate int range.

What’s the difference between php filter sanitize url and php validate url?

php filter sanitize url strips unwanted characters from a URL, while php validate url checks if it follows a proper format. If you’re collecting links, use both together. First, run php sanitize input with php filter sanitize url, then confirm it’s correct using php validate url.

This method supports php input filter data cleaning, php input filter html injection prevention, and php input filter data integrity by catching bad or dangerous input.

How do I use php input filter callback for custom checks?

When built-in filters don’t cover what you need, use a php input filter callback example. This lets you run a custom function through php input filter validate callback. You can add it to a php input filter options array or with php filter default options.

It’s useful for php input filter custom validation when standard tools fall short. It helps you apply more specific php input validation best practices for real-world use.

Why use php filter flags usage and php filter default options when filtering input?

php filter flags usage lets you tweak how filters behave. For example, flags can tell php validate ip to allow IPv6 or php validate boolean to be strict. php filter default options are great for setting fallback values when input is missing.

These features help avoid crashes and apply php input filter error handling. Use them with php input filter data dependency checking or php input filter data enumeration checking for complex forms.

How can I validate things like IPs, domains, and MAC addresses in PHP?

To check network data, use php validate ip, php filter validate domain, or php filter validate mac address. For more control, use php filter validate ipv4 or php filter validate ipv6.

These work well with php input filter data normalization, php input filter data validation, and php input filter data security. They ensure the input looks and behaves like a valid address, keeping your app from accepting fake or harmful values. 

Related Articles

1. https://securecodingpractices.com/secure-coding-in-php/
2. https://securecodingpractices.com/prevent-cross-site-scripting-xss-asp-net-core/
3. https://securecodingpractices.com/secure-file-handling-java-nio/ 

References

• https://www.php.net/manual/en/function.filter-var.php 
• https://www.cisa.gov/sites/default/files/2024-05/Secure_by_Design_Alert_Eliminating_Directory_Traversal_Vulnerabilities_in_Software_508c%20%283%29.pdf