Improve requirements check.

This PR solves a number of issues:
1. The js was only enqueued if the current user had the `activate_plugins` ability (=(super)-admin) rendering the plugin largely useless for multi-author websites where most authors are non-admins.
2. The requirements were checked using `is_plugin_active()` which is unreliable.
    Changed this to a check based on code rather than on the `is_plugin_active()` function.
    This is more reliable as:
    a) it will also take into account plugins loaded through the must-use plugins directory
    b) it will still work even if the user has renamed the folder for the plugin within `wp-content/plugins/` (which happens more often than you'd expect).
3. Allow for *this* plugin to be installed as a must-use plugin, i.e. the admin notice and the deactivation should only happen if the plugin is installed as a normal plugin. So re-arranged the code to deal with that.
4. Get rid of the use of `get_plugin_data()` which is an expensive function if all you really want to do is get the version number of the plugin. Also the original function call was wrong causing a number of errors and failing to retrieve the version number as `dirname( __FILE__ )` was passed, while the function expects `__FILE__`. Replaced with a class constant.

Fixes #6 (point 2)

This PR additionally fixes the following PHP errors which were caused by the bug outlined in point 3:
`PHP Warning:  fopen(/path/to/plugins/yoast-seo-acf-analysis): failed to open stream: Permission denied`
`PHP Warning:  fread() expects parameter 1 to be resource, boolean given in /path/to/wp-includes/functions.php on line 4466`
`PHP Warning:  fclose() expects parameter 1 to be resource, boolean given in /path/to/wp-includes/functions.php on line 4469`

yoast-acf-analysis.php

@@ -21,8 +21,7 @@
  */
 class Yoast_ACF_Analysis {
 
-	/** @var array Plugin information. */
-	private $plugin_data = null;
+	const VERSION = '1.1.0';
 
 	/**
 	 * Yoast_ACF_Analysis constructor.
@@ -44,46 +43,51 @@ function __construct() {
 	 */
 	public function admin_init() {
 
-		// Require ACF and Yoast
-		if ( current_user_can( 'activate_plugins' ) ) {
-			$deactivate = false;
+		$notice_functions = array();
 
-			// ACF
-			if ( ! is_plugin_active( 'advanced-custom-fields/acf.php' ) && ! is_plugin_active( 'advanced-custom-fields-pro/acf.php' ) ) {
-				add_action( 'admin_notices', array( $this, 'acf_not_active_notification' ) );
-				$deactivate = true;
-			}
+		// ACF
+		if ( ! class_exists( 'acf' ) && ! is_plugin_active( 'advanced-custom-fields-pro/acf.php' ) ) {
+			$notice_functions[] = 'acf_not_active_notification';
+		}
 
-			// Yoast SEO for WordPress
-			if ( ! is_plugin_active( 'wordpress-seo/wp-seo.php' ) && ! is_plugin_active( 'wordpress-seo-premium/wp-seo-premium.php' ) ) {
-				add_action( 'admin_notices', array( $this, 'wordpress_seo_requirements_not_met' ) );
-				$deactivate = true;
-			}
-			else {
-				// Compare if version is >= 3.0
-				if ( defined( 'WPSEO_VERSION' ) ) {
-					if ( version_compare( substr( WPSEO_VERSION, 0, 3 ), '3.1', '<' ) ) {
-						add_action( 'admin_notices', array( $this, 'wordpress_seo_requirements_not_met' ) );
-						$deactivate = true;
-					}
+		// Yoast SEO for WordPress
+		if ( ! defined( 'WPSEO_VERSION' ) ) {
+			$notice_functions[] = 'wordpress_seo_requirements_not_met';
+		}
+		// Make sure that version is >= 3.1
+		else if ( version_compare( substr( WPSEO_VERSION, 0, 3 ), '3.1', '<' ) ) {
+			$notice_functions[] = 'wordpress_seo_requirements_not_met';
+		}
+
+		// Stop here if we cannot do the job we are hired to do.
+		if ( ! empty( $notice_functions ) ) {
+			// Deactivate if installed as a plugin.
+			if ( current_user_can( 'activate_plugins' ) && is_plugin_active( plugin_basename( __FILE__ ) ) ) {
+				foreach ( $notice_functions as $function ) {
+					add_action( 'admin_notices', array( $this, $function ) );
 				}
-			}
+				unset( $function );
+
+				$file = plugin_basename( __FILE__ );
+
+				deactivate_plugins( $file, false, is_network_admin() );
 
-			// Deactivate when we cannot do the job we are hired to do.
-			if ( $deactivate ) {
-				deactivate_plugins( plugin_basename( __FILE__ ) );
+				// Add to recently active plugins list.
+				if ( ! is_network_admin() ) {
+					update_option( 'recently_activated', array( $file => time() ) + (array) get_option( 'recently_activated' ) );
+				} else {
+					update_site_option( 'recently_activated', array( $file => time() ) + (array) get_site_option( 'recently_activated' ) );
+				}
 
+				// Prevent trying again on page reload.
 				if ( isset( $_GET['activate'] ) ) {
 					unset( $_GET['activate'] );
 				}
-
-				return;
 			}
-
-			// Only enqueue when we are active.
+		}
+		else {
+			// Only enqueue when all requirements are met.
 			add_filter( 'admin_enqueue_scripts', array( $this, 'enqueue_scripts' ) );
-
-			$this->plugin_data = get_plugin_data( dirname( __FILE__ ) );
 		}
 	}
 
@@ -117,7 +121,7 @@ public function enqueue_scripts() {
 				'jquery',
 				'wp-seo-post-scraper',
 			),
-			$this->plugin_data['Version']
+			self::VERSION
 		);
 
 		// Term page enqueue.
@@ -128,7 +132,7 @@ public function enqueue_scripts() {
 				'jquery',
 				'wp-seo-term-scraper',
 			),
-			$this->plugin_data['Version']
+			self::VERSION
 		);
 	}