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 your application gets
the correct input type.
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 a variable, use one
of the following filter functions:
- filter_var() - Filters 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
In the example below, we
validate an integer using the filter_var() function:
|
<?php
$int = 123;
if(!filter_var($int, FILTER_VALIDATE_INT))
{
echo("Integer is not valid");
}
else
{
echo("Integer is valid");
}
?>
|
The code above uses the
"FILTER_VALIDATE_INT" filter to filter the variable. Since the
integer is valid, the output of the code above will be: "Integer is
valid".
If we try with a variable
that is not an integer (like "123abc"), the output will be:
"Integer is not valid". For a complete list of functions and filters,
visit our PHP Filter Reference.
Validating and Sanitizing
There are two kinds of filters:
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 return 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.
In the example below, we
validate an integer using the filter_var() and the "min_range" and
"max_range" options:
|
<?php
$var=300;
$int_options = array(
"options"=>array
(
"min_range"=>0,
"max_range"=>256
)
);
if(!filter_var($var, FILTER_VALIDATE_INT, $int_options))
{
echo("Integer is not valid");
}
else
{
echo("Integer is valid");
}
?>
|
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".
For a complete list of
functions and filters, visit our PHP Filter Reference. Check each filter to see what options and
flags are available.
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.
In the example below, the input
variable "email" is sent to the PHP page:
|
<?php
if(!filter_has_var(INPUT_GET, "email"))
{
echo("Input type does not exist");
}
else
{
if (!filter_input(INPUT_GET, "email",
FILTER_VALIDATE_EMAIL))
{
echo "E-Mail is not valid";
}
else
{
echo "E-Mail is valid";
}
}
?>
|
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 exist
- 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 the filter_input() function.
In the example below, the
input variable "url" is sent to the PHP page:
|
<?php
if(!filter_has_var(INPUT_POST, "url"))
{
echo("Input type does not exist");
}
else
{
$url = filter_input(INPUT_POST,
"url", FILTER_SANITIZE_URL);
}
?>
|
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.W3ååSchøøools.com/", the $url variable
after the sanitizing will look like this:
|
http://www.W3Schools.com/
|
Filter Multiple Inputs
A form almost always consist
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.
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 e-mail address:
|
<?php
$filters = array
(
"name" => array
(
"filter"=>FILTER_SANITIZE_STRING
),
"age" => array
(
"filter"=>FILTER_VALIDATE_INT,
"options"=>array
(
"min_range"=>1,
"max_range"=>120
)
),
"email"=> FILTER_VALIDATE_EMAIL,
);
$result = filter_input_array(INPUT_GET, $filters);
if (!$result["age"])
{
echo("Age must be a number between 1 and 120.<br
/>");
}
elseif(!$result["email"])
{
echo("E-Mail is not valid.<br />");
}
else
{
echo("User input is valid");
}
?>
|
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" variables in the $result variable for invalid inputs. (If any
of the input variables are invalid, that input variable will be FALSE after the
filter_input_array() function)
The second parameter of the
filter_input_array() function can be an array or a single filter ID.
If the parameter is a single
filter ID all values in the input array are filtered by the specified filter.
If the parameter is an array
it must follow these rules:
- Must be an associative array containing an
input variable as an array key (like the "age" input variable)
- The array value must be a filter ID or an array
specifying the filter, flags and options
Using Filter Callback
It is possible to call a user
defined function and use it as a filter using the FILTER_CALLBACK filter. This
way, we have full control of the data filtering.
You can create your own user
defined function or use an existing PHP function
The function you wish to use
to filter is specified the same way as an option is specified. In an
associative array with the name "options"
In the example below, we use
a user created function to convert all "_" to whitespaces:
|
<?php
function convertSpace($string)
{
return str_replace("_", " ", $string);
}
$string = "Peter_is_a_great_guy!";
echo filter_var($string, FILTER_CALLBACK,
array("options"=>"convertSpace"));
?>
|
The result from the code
above should look like this:
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
