PHP Filters

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PHP filters are used to validate and filter data coming from insecure sources, like user input.

What is a PHP Filter?

A PHP Filter is used to validate and filter data coming from insecure sources.

To test, validate, and filter user input or custom data is an important part of any web application.

The PHP filter extension is designed to make data filtering easier and quicker.

Why use a Filter?

Almost all web applications depend on external input. Usually, this comes from a user or another application  (like a web service). By using filters you can be sure the application gets the correct input.

You should always filter all external data!

Input filtering is one of the most important application security issues.

What is external data?

  • Input data from a form
  • Cookies
  • web services data
  •  Server variables
  •  Database query results

Functions and Filters

To filter available, use one of the following filter function:

  • filter_var()- Filter a single variable with a specified filter.
  • filter_var_array()- Filter several  variables with the same or different filters.
  • filter_input()- Get one input  variable and filter it.
  • filter_input_array – Get several input variables and filter them with the same or different filters.

Example, we validate  an integer using the filter_var() function:

The code above uses the  “FILTER_VALIDATE_INT” filter to filter the variable. Since the integer is valid,

Output:
“Integer is valid”

If we try with a variable that is not an integer (like “a1bc2”), the output will be: “Integer is not valid”.

Validating and Sanitizing

There are two kinds of filter:

Validating Filters:

  • Are used to validate user input
  • Strict format rules (like URL or E-Mail validating)
  • Returns the expected type on success or FALSE on failure

Sanitizing filters:

  • Are used to allow or disallow specified characters in a string.
  • No data format rules.
  • Always returns the string.

Options and Flags

Options and flags are used to add additional filtering options to the specified filters. Different filters have different options and flags.

Example, we  validate an integer using the filter_var() and the “min_range” and “max_range” options:

Like the code above, options must be put in an associative array with the name “options”. If a flag is used it does not need to be in an array.

Since the integer is “300” it is not in the specified range and the output of the code above will be: “Integer is not valid”.

Validate Input

Let’s try validating input from a form.

The first thing we need to do is to confirm that the input data we are looking for exists. Then we filter the input data using the filter_input() function.

Example, the input variable “email” is sent to the PHP page:

Example Explained

The example above has an input (email) sent to it using the “GET” method:

  1. Check if an “email’ input variable of the “GET” type exists.
  2. If the input variable exists, check if it is a valid e-mail address.

Sanitize Input

Let’s try cleaning up an URL sent from a form. First, we confirm that the input data we are looking for exists. Then we sanitize the input data using filter_input() function.

Example below, the input variable  “URL” is sent to  the PHP page:

Example Explained

The example above has an input(URL) sent to it using the “POST” method:

  1. Check if the “URL” input of the “POST” type exists.
  2. If the input variable exists, sanitize  (take away invalid characters) and store it in the $url variable.

If the input variable is a string  like this “http://www.google.com/”, the $url variable after the sanitizing will look like this: http://google.com/

Filter Multiple Inputs

A form almost always consists of more than one input field. To avoid calling the filter_var or  filter_input functions over and over,  we can use the filter_var_array or the filter_input_array functions.

Example:- In this example, we use the filter_input_array() function to filter three GET variables. The received GET variables is a name, an age, and an email addresses:

 
Example Explained

The example above has three inputs(name, age, and email) sent to it using the “GET” method:

  1. Set an array containing the name of input variables and the filters used on the specified input variables.
  2.  Call the filter_input_array() function with the GET input variables and the array we just set.
  3. Check the “age” and “email” variable in the $result variable for invalid inputs. (If any of the input variables are invalid, that input variable will be FALSE after the filter_input_array() function)

The second parameter of the filter_input_array() function can be an array or a single filter ID. If the parameter is a single filter ID all values in the input array are filtered by the specified filter.

If the parameter is an array it must follow these rules:

  • Must be an associative array containing input variables as an array key (like the “age” input variable)
  • The array value must be a filter  ID or an array specifying the filter, flags, and options.

Using Filter Callback

It is possible to call a user-defined function and use it as a filter using the FILTER_CALLBACK filter. This way, we have full control of the data filtering.

You can create your own user-defined function or use an existing PHP function. the function you wish to use to filter is specified the same way as an option is specified. In an associative array with  the name “options”

In the example below, we use a user-created function to convert all “_” to whitespaces:

Output

Hi How are you

Example Explained

The example above  converts all “_” to whitespaces:

  1. Create a function to replace “_” to whitespaces.
  2. Call the filter_var() function with the FILTER_CALLBACK filter and an array containing our function.

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