PHP - Sanitize Input

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In PHP, it is important to ensure that the input data is sanitized properly by removed any undesired characters before it is processed by the server side code. Usually, the users input their data to a PHP web application through a HTML form. If the form data consists of any undesired characters, it may prove to be harmful, hence an appropriate cleansing operation must be performed.

Input sanitization can be done with the help of one or more of the following functions in PHP.

The htmlspecialchars() Function

This function converts special characters to HTML entities.

htmlspecialchars(
   string $string,
   int $flags = ENT_QUOTES | ENT_SUBSTITUTE | ENT_HTML401,
   ?string $encoding = null,
   bool $double_encode = true
): string

In HTML, certain characters have special significance. This htmlspecialchars() function is used to encode special characters in HTML entities. This is useful when you want to display user input as HTML and want to prevent script injection attacks.

The following special characters are translated as shown −

Flag Constants

The flags parameter is a bitmask of one or more of the following flags, which specify how to handle quotes, invalid code unit sequences and the used document type.

Example

Take a look at the following example −

<?php
   $str = 'Welcome To "PHP Tutorial" by <b>TutorialsPoint</b>';
   echo htmlspecialchars($str);
?>

It will produce the following output −

Welcome To "PHP Tutorial" by <b>TutorialsPoint</b>

The strip_tags() Function

The strip_tags() function removes all the HTML and PHP tags from a given string.

strip_tags(string $string, array|string|null $allowed_tags = null): string

This function is very useful when you want ensure that the user input doesn’t contain any potentially malicious tags.

The allowed_tags parameter is an optional second parameter to specify tags which should not be stripped. These are either given as string, or as an array.

Example

Take a look at the following example −

<?php
   $text = '<p>Hello World</p><!-- Comment --> 
      <a href="/test.html">Click Here</a>';
   echo strip_tags($text);
   echo "\n";

   // Allow <p> and <a>
   echo strip_tags($text, '<p><a>');
?>

It will produce the following output −

Hello World 
      Click Here
Hello World

 
      Click Here

The addslashes() Function

The addslashes() function adds backslashes to a string.

addslashes(string $string): string

The function returns a string with backslashes added before characters that need to be escaped. These characters are −

• Single Quote (')

• Double Quote (")

• Backslash (\)

• NUL (The NUL Byte)

Use this function when you are storing user input in a database and want to prevent SQL injection attacks.

Example

Take a look at the following example −

<?php
   $text = "Newton's Laws";
   $str = addslashes($text);  

   // prints the escaped string 
   echo($str);  
?>

It will produce the following output −

Newton\'s Laws

The filter_var() Function

With the help of a specific filter flag, you can use filter_var() function to sanitize user input.

filter_var(mixed $value, int $filter = 
   FILTER_DEFAULT, array|int $options = 0): mixed

The $value parameter is a variable whose value needs to be sanitized. The $filter parameter is any of the predefined filter constants.

Example

The following code shows how you can sanitize Email data −

<?php
   $a = 'abc def@xyz.com';

   $sa = filter_var($a, FILTER_SANITIZE_EMAIL);
   echo "$sa";
?>

It will produce the following output −

abcdef@xyz.com

Example

The following code shows how you can sanitize URLs −

<?php
   $a = "http://example.c o m";

   $sa = filter_var($a, FILTER_SANITIZE_URL);
   echo "$sa";
?>

It will produce the following output −

http://example.com