Fatal error: Call to undefined function plugin_dir_path()

Question

I am trying to create a custom WordPress plugin and I created a new folder inside the "plugins" folder. It's called "wp-services-table". Inside this folder I created two files: wp-services-table.php and wp-services-table-shortcode.php

For now the wp-services-table-shortcode.php file is blank. I only wrote this in the wp-services-table.php file:

<?php
/**
* Plugin Name: Table and Modal Window for Displaying Services
* Description: Displays Services in a table, by Categories and opens a Modal Window when the user clicks for more information.
* Version: 0.1.0
* Author: Ami
*/

//Exit if accessed directly
if ( ! defined( 'ABSPATH' ) ) {
exit; 
}

require_once ( plugin_dir_path(__FILE__) . 'wp-services-table-shortcode.php' );

But I get this error:

Fatal error: Call to undefined function plugin_dir_path() in /home/mysitedomain/public_html/wpfoldername/contentfolder/plugins/wp-services-table/wp-services-table.php on line 14

Could this be because I have a custom name instead of "wp-content"? Or because I installed WordPress in a subfolder ("wpfoldername") and not directly in the "public_html" folder?

Because oterwise I think that the what I wrote in the plugin's file is correct for creating a new plugin.

I also tried to add this:

$dir = plugin_dir_path(__FILE__);
var_dump($dir);
die();

To see what path it would show for the plugin, but of course I still got the same error regarding the undefined function plugin_dir_path()

I hope someone knows why this is happening because I'm very puzzled here. Thank you!

I edited the question to add the contents of wp-config: it might be useful because I have changed the name of the wp-content folder and also WordPress is installed in a subfolder. I think this is the part that is relevant:

/* That's all, stop editing! Happy blogging. */

/** Absolute path to the WordPress directory. */
if ( !defined('ABSPATH') )
define('ABSPATH', dirname(__FILE__) . '/');

/** Renaming the wp-content folder. */
define ('WP_CONTENT_FOLDERNAME', 'contentfolder');
define ('WP_CONTENT_DIR', ABSPATH . WP_CONTENT_FOLDERNAME) ;

define('WP_SITEURL', 'http://' . $_SERVER['HTTP_HOST'] . '/wpfoldername/');
define('WP_CONTENT_URL', WP_SITEURL . WP_CONTENT_FOLDERNAME);

/** Sets up WordPress vars and included files. */
require_once(ABSPATH . 'wp-settings.php');

Also the index.php file is in the "wpfoldername" folder and it contains the following code:

<?php
/**
* Front to the WordPress application. This file doesn't do anything, but loads
* wp-blog-header.php which does and tells WordPress to load the theme.
*
* @package WordPress
*/

/**
* Tells WordPress to load the WordPress theme and output it.
*
* @var bool
*/
define('WP_USE_THEMES', true);

/** Loads the WordPress Environment and Template */
require( dirname( __FILE__ ) . '/wp-blog-header.php' );

Answer 1

Could it be that you forgot to comment the Exit if accessed directly part?

//Exit if accessed directly

if ( ! defined( 'ABSPATH' ) ) {
exit; 
}

Answer 2

How do you access the page? Are you trying to access the pluginfile directly without using the frontend of the site? If that's the case Wordpress probably hasn't been loaded.

What happens if you add the following code at the top of your page?

if ( !defined('ABSPATH') ) {
    //If wordpress isn't loaded load it up.
    $path = $_SERVER['DOCUMENT_ROOT'];
    include_once $path . '/wp-load.php';
}

The code above makes sure Wordpress is loaded before your code runs.