Input validation with filter functions

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Introduction
Although PHP has a lot of filter functions available, I found that still to many people are using (often incorrect) regular expressions to validate user input. The filter extension is simple, standard available and will fulfill the common validations. Below some pratical examples and things to consider when working with PHP filter functions.

Which are available?
Below a shameless copy paste of the PHP documentation.

filter_has_var â€” Checks if variable of specified type exists
filter_id â€” Returns the filter ID belonging to a named filter
filter_input_array â€” Gets external variables and optionally filters them
filter_input â€” Gets a specific external variable by name and optionally filters it
filter_list â€” Returns a list of all supported filters
filter_var_array â€” Gets multiple variables and optionally filters them
filter_var â€” Filters a variable with a specified filter

Pratical use

Sanitizing
“Filter input escape output” every developer knows this but it is a repetitive job but with the filter extension filterering input became a lot easier. When you correctly filter input you drastically lower the change of application vulnerabilities.

Sanitizing a single variable

$sText = ' ';
$sText = filter_var($sText, FILTER_SANITIZE_STRING);
echo $sText; // This is a comment from a alert("scriptkiddie");

Sanitizing multiple variables, same principle as above but with an array, the filter will sanitize all values inside the array

filter_var_array($_POST, FILTER_SANITIZE_STRING);

Validating an email address

if(filter_var($sEmail, FILTER_VALIDATE_EMAIL) === false) {
     $this->addError('Invalid email address', $sEmail);
}

Validation a complete array
Validating all your data at once with a single filter will make your code clear, all in one place and is more easy to maintain an example below.

$aData = array(
	'student'	=> 'Sjoerd Maessen',
	'class'		=> '21',
	'grades' => array(
			'math' => 9,
			'geography' => 66,
			'gymnastics' => 7.5
	)
);

$aValidation = array(
	'student'	=> FILTER_SANITIZE_STRING,
	'class'		=> FILTER_VALIDATE_INT,
	'grades'	=> array(
				'filter' => FILTER_VALIDATE_INT,
				'flags'	 => FILTER_FORCE_ARRAY,
				'options'=> array('min_range'=>0, 'max_range'=>10))
);

echo '';
var_dump(filter_var_array($aData, $aValidation));

/*array(3) {
  ["student"]=>
  string(14) "Sjoerd Maessen"
  ["class"]=>
  int(21) // Thats strange, my string is converted
  ["grades"]=>
  array(3) {
    ["math"]=>
    int(9)
    ["geography"]=>
    bool(false) // 66 is > 10
    ["gymnastics"]=>
    bool(false) // 7.5 is not an int
  }
}*/

Note: okay I did not expect that the string '21' would validate true against FILTER_VALIDATE_INT, after some more testing I also noticed that min_range and max_range only work with FILTER_VALIDATE_INT, when using floats or scalars the options are just ignored, so be aware!
The sanitizing examples above can be made easily more restrictive by adding flags like FILTER_FLAG_STRIP_LOW to the sanitize filter, FILTER_FLAG_STRIP_LOW will for example strip all characters that have a numerical value below 32.
Things to consider

Although the filter functions are some time available some of them aren't flawless, at some points the documentation is missing or very unclear. Another example is the filter_var validation for IPv6 addresses. (see bug report #50117). So it is always a good thing to check if the filter is really doing what you expect it does. Write testcases before using. If you use it correctly you can write your validations in the blink of an eye, and this extension will be your new best friend.
Links

Filter functions

Filter flags