PHP: Sanitize filters

Sanitize filters

**List of filters for sanitization**
ID	Name	Flags	Description
`FILTER_SANITIZE_EMAIL`	"email"		Remove all characters except letters, digits and !#$%&'*+-=?^_`{\|}~@.[].
`FILTER_SANITIZE_ENCODED`	"encoded"	`FILTER_FLAG_STRIP_LOW`, `FILTER_FLAG_STRIP_HIGH`, `FILTER_FLAG_STRIP_BACKTICK`, `FILTER_FLAG_ENCODE_LOW`, `FILTER_FLAG_ENCODE_HIGH`	URL-encode string, optionally strip or encode special characters.
`FILTER_SANITIZE_MAGIC_QUOTES`	"magic_quotes"		Apply addslashes(). (DEPRECATED as of PHP 7.3.0 and REMOVED as of PHP 8.0.0, use `FILTER_SANITIZE_ADD_SLASHES` instead.)
`FILTER_SANITIZE_ADD_SLASHES`	"add_slashes"		Apply addslashes(). (Available as of PHP 7.3.0)
`FILTER_SANITIZE_NUMBER_FLOAT`	"number_float"	`FILTER_FLAG_ALLOW_FRACTION`, `FILTER_FLAG_ALLOW_THOUSAND`, `FILTER_FLAG_ALLOW_SCIENTIFIC`	Remove all characters except digits, `+-` and optionally `.,eE`.
`FILTER_SANITIZE_NUMBER_INT`	"number_int"		Remove all characters except digits, plus and minus sign.
`FILTER_SANITIZE_SPECIAL_CHARS`	"special_chars"	`FILTER_FLAG_STRIP_LOW`, `FILTER_FLAG_STRIP_HIGH`, `FILTER_FLAG_STRIP_BACKTICK`, `FILTER_FLAG_ENCODE_HIGH`	HTML-encode `'"<>&` and characters with ASCII value less than 32, optionally strip or encode other special characters.
`FILTER_SANITIZE_FULL_SPECIAL_CHARS`	"full_special_chars"	`FILTER_FLAG_NO_ENCODE_QUOTES`	Equivalent to calling htmlspecialchars() with `ENT_QUOTES` set. Encoding quotes can be disabled by setting `FILTER_FLAG_NO_ENCODE_QUOTES`. Like htmlspecialchars(), this filter is aware of the default_charset and if a sequence of bytes is detected that makes up an invalid character in the current character set then the entire string is rejected resulting in a 0-length string. When using this filter as a default filter, see the warning below about setting the default flags to 0.
`FILTER_SANITIZE_STRING`	"string"	`FILTER_FLAG_NO_ENCODE_QUOTES`, `FILTER_FLAG_STRIP_LOW`, `FILTER_FLAG_STRIP_HIGH`, `FILTER_FLAG_STRIP_BACKTICK`, `FILTER_FLAG_ENCODE_LOW`, `FILTER_FLAG_ENCODE_HIGH`, `FILTER_FLAG_ENCODE_AMP`	Strip tags and HTML-encode double and single quotes, optionally strip or encode special characters. Encoding quotes can be disabled by setting `FILTER_FLAG_NO_ENCODE_QUOTES`. (Deprecated as of PHP 8.1.0, use htmlspecialchars() instead.)
`FILTER_SANITIZE_STRIPPED`	"stripped"		Alias of "string" filter. (Deprecated as of PHP 8.1.0, use htmlspecialchars() instead.)
`FILTER_SANITIZE_URL`	"url"		Remove all characters except letters, digits and $-_.+!*'(),{}\|\\^~[]`<>#%";/?:@&=.
`FILTER_UNSAFE_RAW`	"unsafe_raw"	`FILTER_FLAG_STRIP_LOW`, `FILTER_FLAG_STRIP_HIGH`, `FILTER_FLAG_STRIP_BACKTICK`, `FILTER_FLAG_ENCODE_LOW`, `FILTER_FLAG_ENCODE_HIGH`, `FILTER_FLAG_ENCODE_AMP`	Do nothing, optionally strip or encode special characters. This filter is also aliased to `FILTER_DEFAULT`.

Warning

When using one of these filters as a default filter either through your ini file or through your web server's configuration, the default flags is set to FILTER_FLAG_NO_ENCODE_QUOTES. You need to explicitly set filter.default_flags to 0 to have quotes encoded by default. Like this:

Example #1 Configuring the default filter to act like htmlspecialchars

filter.default = full_special_chars
filter.default_flags = 0

Changelog

Version	Description
8.1.0	`FILTER_SANITIZE_STRING` and `FILTER_SANITIZE_STRIPPED` have been deprecated.
8.0.0	`FILTER_SANITIZE_MAGIC_QUOTES` has been removed.
7.3.0	`FILTER_SANITIZE_ADD_SLASHES` was added as a replacement for `FILTER_SANITIZE_MAGIC_QUOTES`
7.3.0	`FILTER_SANITIZE_MAGIC_QUOTES` has been deprecated.

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User Contributed Notes 11 notes

down

googlybash24 at aol dot com ¶

11 years ago

Remember to trim() the $_POST before your filters are applied:

<?php

// We trim the $_POST data before any spaces get encoded to "%20"

// Trim array values using this function "trim_value"
function trim_value(&$value)
{
$value = trim($value);    // this removes whitespace and related characters from the beginning and end of the string
}
array_filter($_POST, 'trim_value');    // the data in $_POST is trimmed

$postfilter =    // set up the filters to be used with the trimmed post array
array(
'user_tasks'                        =>    array('filter' => FILTER_SANITIZE_STRING, 'flags' => !FILTER_FLAG_STRIP_LOW),    // removes tags. formatting code is encoded -- add nl2br() when displaying
'username'                            =>    array('filter' => FILTER_SANITIZE_ENCODED, 'flags' => FILTER_FLAG_STRIP_LOW),    // we are using this in the url
'mod_title'                            =>    array('filter' => FILTER_SANITIZE_ENCODED, 'flags' => FILTER_FLAG_STRIP_LOW),    // we are using this in the url
);

$revised_post_array = filter_var_array($_POST, $postfilter);    // must be referenced via a variable which is now an array that takes the place of $_POST[]
echo (nl2br($revised_post_array['user_tasks']));    //-- use nl2br() upon output like so, for the ['user_tasks'] array value so that the newlines are formatted, since this is our HTML <textarea> field and we want to maintain newlines
?>

down

Anonymous ¶

8 years ago

FILTER_SANITIZE_STRING doesn't behavior the same as strip_tags function.    strip_tags allows less than symbol inferred from context, FILTER_SANITIZE_STRING strips regardless.
<?php
$smaller = "not a tag < 5";
echo strip_tags($smaller);    // -> not a tag < 5
echo filter_var ( $smaller, FILTER_SANITIZE_STRING); // -> not a tag
?>

down

ipse at sergiosantos dot me ¶

3 years ago

Although it's specifically mentioned in the above documentation, because many seem to find this unintuitive it's worth pointing out that FILTER_SANITIZE_NUMBER_FLOAT will remove the decimal character unless you specify FILTER_FLAG_ALLOW_FRACTION:

<?php
$number_string = '12.34';

echo filter_var( $number_string, FILTER_SANITIZE_NUMBER_FLOAT ); // 1234

echo filter_var( $number_string, FILTER_SANITIZE_NUMBER_FLOAT, FILTER_FLAG_ALLOW_FRACTION ); // 12.34
?>

down

finkenb2 at msu dot edu ¶

8 months ago

With the deprecation of FILTER_SANITIZE_STRING, the "use htmlspecialchars instead" is an incomplete comment.  The functionality of FILTER_SANITIZE_STRING was a combination of htmlspcialchars and (approximately) strip_tags.  For true compatibility a polyfil may be needed:

<?php
function filter_string_polyfill(string $string): string
{
$str = preg_replace('/\x00|<[^>]*>?/', '', $string);
    return str_replace(["'", '"'], ['&#39;', '&#34;'], $str);
}

$string = "Some \"' <bizzare> string & to Sanitize < !$@%";
echo filter_var($string,FILTER_SANITIZE_STRING).PHP_EOL;
//Some &#34;&#39;  string & to Sanitize 

echo htmlspecialchars($string).PHP_EOL;
//Some &quot;&#039; &lt;bizzare&gt; string &amp; to Sanitize &lt; !$@%

echo strip_tags($string).PHP_EOL;
//Some "'  string & to Sanitize < !$@%

echo htmlspecialchars(strip_tags($string,ENT_QUOTES)).PHP_EOL;
//Some &quot;&#039;  string &amp; to Sanitize &lt; !$@%

echo filter_string_polyfill($string).PHP_EOL;
//Some &#34;&#39;  string & to Sanitize

down

Willscrlt ¶

8 years ago

To include multiple flags, simply separate the flags with vertical pipe symbols.

For example, if you want to use filter_var() to sanitize $string with FILTER_SANITIZE_STRING and pass in FILTER_FLAG_STRIP_HIGH and FILTER_FLAG_STRIP_LOW, just call it like this:

$string = filter_var($string, FILTER_SANITIZE_STRING, FILTER_FLAG_STRIP_HIGH | FILTER_FLAG_STRIP_LOW);

The same goes for passing a flags field in an options array in the case of using callbacks.

$var = filter_var($string, FILTER_SANITIZE_SPECIAL_CHARS,
array('flags' => FILTER_FLAG_STRIP_LOW | FILTER_FLAG_ENCODE_HIGH));

Thanks to the Brain Goo blog at popmartian.com/tipsntricks/for this info.

down

marcus at synchromedia dot co dot uk ¶

14 years ago

It's not entirely clear what the LOW and HIGH ranges are. LOW is characters below 32, HIGH is those above 127, i.e. outside the ASCII range.

<?php
$a = "\tcafé\n";
//This will remove the tab and the line break
echo filter_var($a, FILTER_SANITIZE_STRING, FILTER_FLAG_STRIP_LOW);
//This will remove the é.
echo filter_var($a, FILTER_SANITIZE_STRING, FILTER_FLAG_STRIP_HIGH);
?>

down

AntonioPrimera ¶

8 years ago

Please be aware that when using filter_var() with FILTER_SANITIZE_NUMBER_FLOAT and FILTER_SANITIZE_NUMBER_INT the result will be a string, even if the input value is actually a float or an int.

Use FILTER_VALIDATE_FLOAT and FILTER_VALIDATE_INT, which will convert the result to the expected type.

down

galvao at galvao dot eti dot br ¶

11 years ago

Just to clarify, since this may be unknown for a lot of people:


ASCII characters above 127 are known as "Extended" and they represent characters such as greek letters and accented letters in latin alphabets, used in languages such as pt_BR.


A good ASCII quick reference (aside from the already mentioned Wikipedia article) can be found at: http://www.asciicodes.com/

down

david dot drakulovski at gmail dot com ¶

10 years ago

Here is a simpler and a better presented ASCII list for the <32 or 127> filters 
(if wikipedia confused the hell out of you):

http://www.danshort.com/ASCIImap/

down

terry dot marsault at gmail dot com ¶

3 months ago

None of these parameters do answer my need, so I've made my own function, hope it helps some other folks!

function removeSpecialChars($valueToClean)
{
    return htmlspecialchars(str_replace([",", "#", "$", "%", "*", "~", "'", "=", "{", "[", "|", "`", "^", "]", "}", ":", ";", "<", ">", "/", "?", "&"], "", $valueToClean));
}

down

-2

darren at daz-web dot com ¶

5 years ago

For those looking for a simple way around filtering POST forms that have textarea elements in them. If you also need tab for example you can extend quite easily.

<?php 
//create an array of all relevant textareas
$textareas = array("ta1");

foreach($_POST as $k => $v)
    {
$v = trim($v);//so we are sure it is whitespace free at both ends

        //preserve newline for textarea answers
if(in_array($k,$textareas))$v=str_replace("\n","[NEWLINE]",$v); 

//sanitise string
$v = filter_var($v, FILTER_SANITIZE_STRING, FILTER_FLAG_STRIP_LOW | FILTER_FLAG_STRIP_HIGH | FILTER_FLAG_STRIP_BACKTICK);

//now replace the placeholder with the original newline
$_POST[$k] = str_replace("[NEWLINE]","\n",$v);
    }

//simple form for testing submital

?><!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Filter test</title>
</head>

<body>

<form action="" method="post">
    <p>
        <textarea name="ta1" cols="30" rows="10"><?php echo $_POST['ta1']; ?></textarea>
    </p>
    <p>
        <input type="text" name="txt1" size="30" value="<?php echo $_POST['txt1']; ?>" />
    </p>
    <p>
        <input type="submit" />    
    </p>
    </form>

</body>

</html>

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