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valid-array

Data filtering mechanism based on native php-filter extension with additional capacities.

A ValidArray object receives on instantiation an associative array with elements names as keys and elements filters as values. These filters guarantee that when corresponding elements in this object are set, they will automatically get validated and their valid value will be stored. If an element does not pass validation, a default value may be used basing on filter parameters. Object may be used as a normal array to set elements and get elements values, validation is done on element modification.

ValidArray extends SPL ArrayObject and wraps the functionality of php-filter extension.

Introduction

PHP native data validation mechanism using standard php-filter extension is a great way of working with data, to ensure user input correctness and to protect against several attack vectors. This extension is bundled with PHP and is available by default in most configurations, which makes it a natural choice for any project requiring data validation.

As a bundled extension, php-filter benefits from documentation coverage in PHP manual, performance gain of compiled-in module and quasi-absolute availability.

While being a de-facto standard for data validation in PHP, it still suffers from several questionable architectural decisions that were made at the early days and that may slow down the learning curve and development pace when implementing validation strategy in user land.

Some of these dubious decisions:

• 🤨 default value is implemented for limited number of filters, only to replace improperly formatted arguments that otherwise match defined filter flags, but not for cases when argument is not supplied or when flags mismatch is detected,
• 🤔 limited FILTER_CALLBACK functionality, when flags are ignored and validation callback cannot distinguish scalar data from array,
• 🧐 objects may be filtered, but only if they have a magic method __toString() defined (implement Stringable since v8.0), and only this value is kept as element value after validation.

Based on numerous strengths of php-filter extension, ValidArray class corrects (where possible) beforementioned flaws and implements php-filter functionality into a useful and predictable data structure with the following features:

• associative arrays with validation mechanism,
• validation is provided on array instantiation and item modification,
• filters are set at data object instantiation,
• useful for input / output parameters validation,
• minimal footprint,
• extending ArrayObject allows for natural array handling in user code,
• use of standard php-filter extension with enhanced handling of default values, callbacks and objects filtering.

Example

Enough talks, let's see how it works.

We shall handle a login POST request with email, password and uri parameters that need validation.

The route POST /api/login will call LoginController and pass request parameters in an array which should have 3 fields: email, password and uri. All fields should have valid data, which is defined as follows:

• email is a string with valid email address or false if not valid,
• password is a string with hashed version of sent password (calculated during validation) or false if not valid; valid password is a string of at least 8 characters,
• uri is a string with a path for next page to redirect after login or #member by default.

We shall identify request parameters inside the corresponding controller, like in the following simplified code, where our goal will be to implement withRequestVars() method:

Implementing required validation with php-filter (1st attempt):

If you need help with using php-filter extension have a look at official documentation.

Now, this will partly work. Until someone discovers out login api and decides to bring our system down with several absolutely legitimate requests of the following form (newlines added for readability):

When php-filter extension sees an array sent, its default behavior is to validate every item in this array via the provided filter and keep this array structure in place replacing array values with filtered values. In most cases this may be a correct behavior (anyway that's how filter extension works), but in our case this will start calculating password hashes for all the provided passwords, and this is a very CPU intensive task. A couple of dozens of such requests sent in parallel is enough to put our infra in trouble.

php-filter has a very elegant solution for this situation, which is called filter flags, and our first intention will be to set the FILTER_REQUIRE_SCALAR.

Set FILTER_REQUIRE_SCALAR flag (2nd attempt):

Nothing wrong with this approach, and scalar flag will work fine for email and uri fields, but not for the password field. Yes, FILTER_CALLBACK simply ignores flags by design. You cannot do anything with it, so before the validation step you'll likely want to add a verification to catch the possibility of array passed as password argument.

Other thing, and also up to you to handle correctly, is the fact that default value provided via the default flag, is only used when the parameter is provided but is not valid. It is only available for FILTER_VALIDATE_* filters (not for FILTER_DEFAULT by the way), it will not be used if any sanitize-filter shrinks parameter value down to an empty string, if parameter value mismatches defined flags, and finally, it will not be used if parameter was not set in request.

So our final version should handle these additional conditions.

Handle array type and default value manually (3rd attempt):

This is definitely not the most beautiful piece of code and that is where ValidArray is improving the situation.

Consider the same example, but implemented using ValidArray functionality:

Implementing required validation with ValidArray:

ValidArray specificity

Notable distinctions between ValidArray and filter_*() functions:

• ValidArray is an object but allows array-style access to elements since extending ArrayObject SPL class;
• it sets default values or null for missing elements for all filters;
• it provides ValidArray::FILTER_EXTENDED_CALLBACK filter allowing to define callbacks that unlock both flags and default functionality;
• it provides ValidArray::FILTER_INSTANCE_OF filter allowing to validate values as objects of specific type and use both flags and default functionality;
• ValidArray always uses $add_empty mode of filter_var_array() function and does not allow to unset elements that have filters defined;
• count() on ValidArray object will always return the number of defined filters;
• setting value for undefined key will be ignored;
• setting value for existing key will trigger a filter_var() call, which may modify the value in the following ways:
  • change value (ex: FILTER_SANITIZE_NUMBER_INT),
  • change the type of the value (ex: FILTER_VALIDATE_INT),
  • convert scalar value to array (with FILTER_FORCE_ARRAY flag),
  • set value to its default value, false or null in case of failure (attention, not all standard filters use default value even when one is provided),
  • if value is an array (or becomes an array via FILTER_FORCE_ARRAY flag), these modifications will be applied to all array elements, recursively;
• unsetting value will set it to either default value (if defined) or null for all filters.

More use cases

In a specific controller we handle the following request parameters:

So we define the following filters:

We then create a ValidArray instance passing there predefined $filters and the value of the $_REQUEST (which normally accumulates values of GET, POST and COOKIE parameters):

Now we can be sure that the count of $va array is exactly 4 (id, name, email, tel keys), that each of them is of corresponding type or null if not provided in request or false if incoming value does not correspond to definition in filter. The default value for tel element will be used if not provided or invalid.

Since ValidArray extends ArrayObject, its elements may be accessed (added, iterated etc.) via normal array notation:

When setting ValidArray values, they are automatically validated using the predefined filters. Here again, if provided value does not pass validation and no default value is defined, false is set instead (or null if FILTER_NULL_ON_FAILURE flag is set).

More examples

For the following request parameters:

$va contents will be:

For incorrect request parameters:

$va contents will be:

Here, correct parameters will be present, all unknown parameters will be ignored, missing parameters will be set to null or provided default value, and incorrect parameters will be set to false.

Additional filters

These additional filters are provided to enhance standard php-filter capacities:

• ValidArray::FILTER_EXTENDED_CALLBACK
• ValidArray::FILTER_INSTANCE_OF

IMPORTANT

Filters in ValidArray object are read-only and cannot change during object lifespan. This is by design. If you need object with updatable filters, use provided MutableValidArray object instead.