Wordpress functions give fatal error in included php file

Question

First, my problem is similar to this thread. However, the solution posted there doesn't work for me.

I'm creating a plugin that displays a contact form on a page using the Shortcode API.

Plugin folder contents:

• contact.php (main plugin file)
• validate.php (included php file)

The main file has the usual plugin header, and WordPress functions called in it work as expected [i.e: get_permalink()]

The validate.php file is included in contact.php as follows:

define ("PLUGIN_PATH", plugin_dir_path(__FILE__));
include_once dirname( __FILE__ ) . '/validate.php';

My problem is any WordPress functions called in validate.php will throw a fatal error. PHP functions work as expected, only WordPress functions are throwing a fatal error:

Fatal error: Call to undefined function sanitize_text_field() in /{FULL WEBSITE PATH}/ralrom-contactform/validate.php 

How should I be including the validate.php file to be able to call WordPress functions from it?

I have tried "require" and "include" variations and all throw the same fatal error.

Before using multiple files, I had a working version as a single file shown here:

Orignal working code under a single php file: http://pastebin.com/qM5aj9qY

Broken code under two php files: http://pastebin.com/KUCbg6rJ

Answer 1

You're POSTing the form to the validate.php file directly, so the WordPress code isn't loaded. Instead you need to POST the form to the current URL or the home_url or something like that, and then have the plugin intercept the data and act accordingly, so that the WordPress code is loaded before you use its functions.

Answer 2

Thanks to the comments I figured out that what I was trying to do wasn't exactly possible.

The reason being POSTing data to a php page will send the POST data outside of the WordPress loop or "framework". Therefore the WordPress functions aren't loaded. A way around this is to require the wordpress files that contain the functions you want to use inside your php page, i.e:

require_once {path to wordpress wp-includes}/formatting.php;

However, I felt this was a dirty way to do things.

Instead, I followed Otto's suggestion and intercepted the POST data before WordPress loaded the page by attaching a function to the "init" hook:

add_action("init", "intercept_post");

The form action is set to the current page and in the intercept_post function I run data validation, send the email and redirect to the same page. This effectively sends the POST data and prevents a page refresh from submitting the data again.