Filtering and Sanitizing

Sanitizing user input is a critical part of software development. Trusting or neglecting to sanitize user input could lead to unauthorized access to the content of your application, mainly user data, or even the server your application is hosted on.

Full image (from xkcd)

The Phalcon\Filter component provides a set of commonly used filters and data sanitizing helpers. It provides object-oriented wrappers around the PHP filter extension.

Types of Built-in Filters

The following are the built-in filters provided by this component:

Name	Description
string	Strip tags and escapes HTML entities, including single and double quotes.
email	Remove all characters except letters, digits and !#$%&*+-/=?^_`{|}~@.[].
int	Remove all characters except digits, plus and minus sign.
float	Remove all characters except digits, dot, plus and minus sign.
alphanum	Remove all characters except [a-zA-Z0-9]
striptags	Applies the strip_tags function
trim	Applies the trim function
lower	Applies the strtolower function
upper	Applies the strtoupper function

Sanitizing data

Sanitizing is the process which removes specific characters from a value, that are not required or desired by the user or application. By sanitizing input we ensure that application integrity will be intact.

use Phalcon\Filter;

$filter = new Filter();

// Returns "[email protected]"
$filter->sanitize("some(one)@exa\mple.com", "email");

// Returns "hello"
$filter->sanitize("hello<<", "string");

// Returns "100019"
$filter->sanitize("!100a019", "int");

// Returns "100019.01"
$filter->sanitize("!100a019.01a", "float");

Sanitizing from Controllers

You can access a Phalcon\Filter object from your controllers when accessing GET or POST input data (through the request object). The first parameter is the name of the variable to be obtained; the second is the filter to be applied on it.

use Phalcon\Mvc\Controller;

class ProductsController extends Controller
{
    public function indexAction()
    {

    }

    public function saveAction()
    {
        // Sanitizing price from input
        $price = $this->request->getPost("price", "double");

        // Sanitizing email from input
        $email = $this->request->getPost("customerEmail", "email");
    }
}

Filtering Action Parameters

The next example shows you how to sanitize the action parameters within a controller action:

use Phalcon\Mvc\Controller;

class ProductsController extends Controller
{
    public function indexAction()
    {

    }

    public function showAction($productId)
    {
        $productId = $this->filter->sanitize($productId, "int");
    }
}

Filtering data

In addition to sanitizing, Phalcon\Filter also provides filtering by removing or modifying input data to the format we expect.

use Phalcon\Filter;

$filter = new Filter();

// Returns "Hello"
$filter->sanitize("<h1>Hello</h1>", "striptags");

// Returns "Hello"
$filter->sanitize("  Hello   ", "trim");

Combining Filters

You can also run multiple filters on a string at the same time by passing an array of filter identifiers as the second parameter:

use Phalcon\Filter;

$filter = new Filter();

// Returns "Hello"
$filter->sanitize(
    "   <h1> Hello </h1>   ",
    [
        "striptags",
        "trim",
    ]
);

Creating your own Filters

You can add your own filters to Phalcon\Filter. The filter function could be an anonymous function:

use Phalcon\Filter;

$filter = new Filter();

// Using an anonymous function
$filter->add(
    "md5",
    function ($value) {
        return preg_replace("/[^0-9a-f]/", "", $value);
    }
);

// Sanitize with the "md5" filter
$filtered = $filter->sanitize($possibleMd5, "md5");

Or, if you prefer, you can implement the filter in a class:

use Phalcon\Filter;

class IPv4Filter
{
    public function filter($value)
    {
        return filter_var($value, FILTER_VALIDATE_IP, FILTER_FLAG_IPV4);
    }
}

$filter = new Filter();

// Using an object
$filter->add(
    "ipv4",
    new IPv4Filter()
);

// Sanitize with the "ipv4" filter
$filteredIp = $filter->sanitize("127.0.0.1", "ipv4");

Complex Sanitizing and Filtering

PHP itself provides an excellent filter extension you can use. Check out its documentation: Data Filtering at PHP Documentation

Implementing your own Filter

The Phalcon\FilterInterface interface must be implemented to create your own filtering service replacing the one provided by Phalcon.