WordPress Data Validation Functions

As a developer of WordPress plugins or themes, you need to be aware of and use data validation.  What is data validation you ask?  It’s when you make sure that the data you fetched (POST, GET, database call, external source) is the type of data that you expected.  For instance, let’s say you have a user enter a number between 1-10.  They enter the letter ‘A’.  The process of determining whether the input is an integer between 1-10 is data validation.

So what kind of data functionality does WordPress (and PHP in general) offer?  Lots!

• intval($value) – This will cast any value as an integer.  Particularly useful for casting floating point numbers.
• absint($value) – Returns the absolute value of a number.  For those of you with no math background, that means it will return a whole number given any floating point number.  (Ex.  absint(3.3) = 3)
• wp_kses() – This function will strip a string of any HTML tags that are not allowed.  It also makes sure that any HTML entities that are in the string are normal.
• esc_html($string) – This will escape any HTML characters in a string.  This is handy for storing blogs of HTML in a database.
• esc_js($string) – Escapes any javascript that it is given.  Mostly this means it escapes single and double quotes.
• urlencode($string) – This function encodes any string you put in to it as a url-safe value.
• $wpdb->prepare() – This function is used prepare SQL statements for database inserts.  I wrote an article about using the WPDB class with your plugin that you should check out.
• validate_file(..) – This  is useful to validate that a file exists, and also to help prevent directory traversal attacks.
• wp_redirect() – This is the safest was to do redirects.  Instead of using header(Location: ..), this will only allow redirects to white listed domains instead domains.
• balanceTags($string) – If you’re allowing your users to comment on something with html tags, this function will try to make sure that the tags are balanced.
• is_email($email) – Validates whether an email address is valid or not.

As you can see, WordPress (and PHP) include a nice array of data validation functions.  Make sure you use them as often as possible, because a large number of web based attacks could be prevented if people validated data.