Fatal error: Call to undefined function wp_insert_post()

Question

So I have a form that has makes the action jump to another php page.

There my php code tries to upload a post. I get the following error:

Fatal error: Call to undefined function wp_insert_post()

PHP Code:

ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);

if( 'POST' == $_SERVER['REQUEST_METHOD'] && !empty( $_POST['action'] ) &&  $_POST['action'] == "upload_ugc") {

if($_POST["editindication"] == 'newpost'){
$post_array = array(
    'post_title'    => $_POST["post_title"],
    'post_content'  => '',
);
$post_id = wp_insert_post( $post_array, true );
}

Answer 1

If you'd like to use WordPress functions inside your naked PHP file, you'll have to load the WordPress environment:

require_once '/path/to/wp-load.php';

That said it's best practice to NOT handle things via separate PHP files like that outside of WordPress, but use a plugin instead and fire your code on an appropriate action. You can use a page template or a shortcode to render your form, and then catch the form input on the init or template_redirect hook from within a plugin:

add_action( 'init', function() {
    if ( empty( $_POST['action'] ) || $_POST['action'] != 'upload_ugc' )
        return;

    // Your wp_insert_post statement
    wp_redirect( home_url( '/upload-success/' ) );
    die();
});

Also, when working with user input, don't forget about security, especially for logged in users. In your case you'll need a wp_nonce_field at least, and probably run your post title and content through wp_kses.