Add a check for functions ending in `__premium_only`

[monzuralam]

Summary

Plugin Check does not currently flag PHP function declarations whose names end in __premium_only.

This naming pattern is often used to mark code that belongs to a paid/pro/premium version of a plugin. If this code is included in a WordPress.org plugin submission, Plugin Check should report it so authors can remove or relocate the premium-only code before review.

Problem

When a plugin contains functions such as:

function my_plugin_register_settings__premium_only() {
	register_setting( 'my_plugin_settings', 'my_plugin_enabled' );
}

function my_plugin_admin_notice__premium_only() {
	echo '<div class="notice notice-info"><p>Premium-only feature enabled.</p></div>';
}

Plugin Check currently does not identify these declarations as potential premium/pro-only code.

Expected behavior

Plugin Check should flag function declarations ending in __premium_only, for example:

function example_feature__premium_only() {}

Suggested result:

• Type: Error, or warning if the project prefers a lower severity first
• Category: Plugin Repo
• Code: PremiumOnlyFunction.Found
• Message: Function declarations ending in __premium_only look like premium/pro-only code and should not be included in a WordPress.org plugin.

Current behavior

Plugin Check completes without reporting the __premium_only function names.

Why this matters

WordPress.org plugins should not include code intended only for a paid/pro/premium version. Detecting common naming conventions like __premium_only would help catch this issue earlier and make Plugin Check more useful before plugin submission.

Reproduction steps

1. Create a test plugin with this file:

<?php
/**
 * Plugin Name: Premium Only Function Test
 * Description: Test plugin containing functions named with the __premium_only suffix.
 * Author: Monzur Alam
 * Version: 1.0.0
 */

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

function poft_register_settings__premium_only() {
	register_setting( 'poft_settings', 'poft_enabled' );
}

function poft_admin_notice__premium_only() {
	echo '<div class="notice notice-info"><p>Premium-only test notice.</p></div>';
}

2. Run Plugin Check against the test plugin.
3. Observe that no issue is reported for the __premium_only function declarations.

Proposed implementation

Add a static file check that scans PHP files for function declarations ending in __premium_only.

Possible regex:

/\bfunction\s+[a-zA-Z_\x80-\xff][a-zA-Z0-9_\x80-\xff]*__premium_only\s*\(/

The check could extend Plugin Check's existing file-check infrastructure:

WordPress\Plugin_Check\Checker\Checks\Abstract_File_Check

and register under the existing Plugin Repo category.