Wiley Professional WordPress Plugin Development
Contents
1. Plugins Plugins screen Here you can find a list of all plugins active or not available in your WordPress installation You can easily activate deactivate edit update and delete plugins from this screen The Plugin screen also features bulk actions for activating deactivating updating and deleting plugins Check all the plugins you want to manage and then select the appropriate bulk action from the drop down menu This process makes managing multiple plugins a breeze Editing Plugins WordPress features a built in plugin editor under the Plugins Editor screen The plugin editor enables you to view and edit the source code of any plugin installed in WordPress Keep in mind you can only edit the source code if the plugin file is writeable by the web server otherwise you can only view the code To use the editor select the plugin from the drop down menu on the top left portion of the Edit Plugins page The editor lists all files associated with the selected plugin There is also a documentation lookup feature making it easy to research a specific function s purpose in the plugin you are reviewing A word of caution when using the built in plugin editor A browser doesn t have an Undo button There is also no code revision history so one bad code edit can crash your entire site with no way to revert the changes back It s best to use the code editor for reference only and never use it to edit your plugin files Plugin Di2. are also a common feature found in themes today Easy Updates WordPress makes it easy to update a plugin to the latest version Every plugin installed from the WordPress org Plugin Directory alerts you when a new version of the plugin has been released Updating the plugin is as simple as clicking the update notification listed just below the plugin details on the Plugin screen Plugins not installed from the Plugin Directory can also be updated using the auto update functionality of WordPress The plugin author must define where WordPress can download the latest version and it will take care of the rest If the plugin author doesn t define this location you must manually update the plugin Keeping plugins updated is an important part in keeping your web site free from security vulnerabilities and bugs Installing and Managing Plugins 7 Easier to Share and Reuse Plugins are easy to share with others It s much easier to share a plugin than tell someone to modify specific lines of code in your theme or WordPress Using plugins also makes it easy to use the same functionality across multiple sites If you find a group of plugins that you like you can easily install them on every WordPress web site you create Plugin Sandbox When you activate a broken plugin in WordPress it won t break your site If the plugin triggers a fatal error WordPress automatically deactivates the plugin before it has a chance to This fail safe featu
3. major advantages to using plugins and why plugin functionality shouldn t always be included in a theme was discussed Installing and managing plugins in the WordPress admin dashboard was covered Now that you understand how plugins work in WordPress it s time to create the plugin foundation
4. restrict your code to execute only if a user is an administrator Look at an example lt php if current_user_can manage_options any code entered here will only be executed IF user is an administrator gt As you can see it s easy to verify a user has proper permissions prior to executing any code in your plugin You learn about user accounts and roles in Chapter 8 Users As another example look at sending an email in WordPress Sure you could create a new function in your plugin to send email but why WordPress has a handy function called wp_mai1 for sending email Look at an example lt php Semail_to you example com Semail_ subject Plugin email example Semail_ message How do you like my new plugin wo_mail Semail_to Semail_subject Semail_message gt 6 CHAPTER1 AN INTRODUCTION TO PLUGINS As you can see sending an email in WordPress couldn t be easier Unless your plugin needs some customized emailing functionality you don t need to re create this function from scratch Using this function also ensures the widest adoption for sending emails from WordPress because you use the built in function Using the available built in features of WordPress can greatly reduce the time to develop a plugin Another advantage to not reinventing the wheel is that this approach more often than not will allow for your plugins to work across a greater number of servers and setups t
5. An Introduction to Plugins WHAT S IN THIS CHAPTER Understanding a plugin Using available WordPress APIs Loading order of plugins Finding examples of popular plugins Determining the separation of plugin and theme functionality Managing and installing plugins Y Y VV VY VY VY Understanding types of WordPress plugins WordPress is one of the most popular open source content management systems available today One of the primary reasons WordPress is so popular is the ease with which you can customize WordPress through plugins WordPress has an amazing framework in place giving plugin developers the tools needed to extend WordPress in any way imaginable Understanding how plugins work and the tools available in WordPress is critical knowledge when developing professional WordPress plugins WHAT IS A PLUGIN A plugin in WordPress is a PHP script that extends or alters the core functionality of WordPress Quite simply plugins are files installed in WordPress to add a feature or set of features to WordPress Plugins can range in complexity from a simple social networking plugin to an extremely elaborate e commerce package There is no limit to what a plugin can do in WordPress because of this there is no shortage of plugins available for download 2 CHAPTER1 AN INTRODUCTION TO PLUGINS How Plugins Interact with WordPress WordPress features many different APIs for use in your plugin Each API or application programming int
6. Galleries can be embedded in posts or pages gt http wordpress org extend plugins nextgen gallery Contact Form 7 Adds a contact form to any post or page in WordPress Supports mul tiple contact forms Akismet spam filtering and CAPTCHA gt http wordpress org extend plugins contact form 7 As you can see the preceding plugins can handle any task The features added by these plugins are universal and features that most web sites on the Internet should have Popular Plugin Tags Now you will look at some popular tags for plugins Plugin tags are just like blog post tags simple keywords that describe a plugin in the Plugin Directory This makes it easy to search for existing plugins by tag Following are popular examples gt Twitter Everyone loves Twitter for micro blogging and sharing links You can find an abundance of Twitter related plugins for WordPress gt http wordpress org extend plugins tags twitter Google With so many different services and APIs Google is a popular plugin tag Everything from Google ads to Google maps have been integrated into a WordPress plugin gt http wordpress org extend plugins tags google Widget Most plugins that include a widget also use the widget tag This is great for viewing the many different types of widgets available for WordPress gt http wordpress org extend plugins tags widget Advantages of Plugins 5 Viewing popular plugin tags is a great w
7. ay to get inspiration when developing new plugins for WordPress ADVANTAGES OF PLUGINS WordPress offers many advantages to using plugins You need to understand the advantages to building plugins to truly understand why you should build plugins This can also help when determining the need for a specific plugin in WordPress Not Modifying Core One of the main advantages to plugins is the ability to modify the behavior of WordPress without modifying any core files Core files refer to any file that is a part of the default WordPress installation Hacking core files can make it difficult to update WordPress when a new version is released If you made any modifications to a core file that modification would be overwritten when the update occurs Keeping WordPress up to date with the latest version is essential in keeping your web site secure Modifying core files can also lead to an unstable web site Different areas of WordPress rely on other areas to function as expected If you modify a core file and it no longer works as expected it can cause instability and quite possibly break a completely unrelated feature in WordPress Why Reinvent the Wheel Another advantage to building plugins is the structure that already exists for your plugin Many of the common features have already been developed and are ready for use in your plugin For example you can take advantage of the built in user roles in WordPress Using the user roles you can easily
8. erface helps interact with WordPress in a different way Following is a list of the main available APIs in WordPress and their function gt Plugin Provides a set of hooks that enable plugins access to specific parts of WordPress WordPress contains two different types of hooks Actions and Filters The Action hook enables you to trigger custom plugin code at specific points during execution For example you can trigger a custom function to run after a user registers a user account in WordPress The Filter hook to modifies text before adding or after retrieving from the database Widgets Create and manage widgets in your plugin Widgets appear under the Appearance Widgets screen and are available to add to any registered sidebar in your theme The API enables multiple instances of the same widget to be used throughout your sidebars Shortcode Adds shortcode support to your plugin A shortcode is a simple hook that enables you to call a PHP function by adding something such as shortcode to a post or page HTTP Sends HTTP requests from your plugin This API retrieves content from an external URL or for submitting content to a URL Currently you have five different ways to send an HTTP request This API standardizes that process and tests each method prior to executing Based on your server configuration the API will use the appropriate method and make the request Settings Inserts settings or a settings section for your p
9. g SMTP rather than the default method wp config Loaded All pluggable functions are defined in the wo includes pluggable php tb Core WordPress file Functions Loaded You can use some predefined functions during specific plugin tasks such as when a plugin is activated or deactivated and even when a plugin v vy is uninstalled Chapter 2 Plugin Foundation covers these functions Plugins Loaded in detail i Pluggables Loaded When Are Plugins Loaded uggables Loade Plugins are loaded early in the process when a WordPress powered web page is called Figure 1 1 shows a diagram of the standard loading process when loading a page in WordPress Translations Loaded Figure 1 1 illustrates the standard process when loading a page in Theme Loaded WordPress The flow changes slightly when loading an admin page The differences are minor and primarily concern what theme is loaded admin theme versus your web site theme Page Content Lh po _ Page Content FIGURE 1 1 AVAILABLE PLUGINS When researching available plugins you need to know where to find WordPress plugins You can download plugins anywhere on the Internet but this isn t always a good idea As with any software downloading plugins from an untrusted source could lead to malware injected and compromised plugin files It s best to download plugins only from trusted web sites and official sources such as the official Plugin Directory Official P
10. hereby maximizing compatibility Don t reinvent the wheel with features that already exist in WordPress Separating Plugins and Themes A plugin can take control of the rendering process therefore the plugin can become a theme Similarly a theme can have plugin functionality included Because of this the difference between the two can sometimes become blurred so why not just include your plugin code directly in a theme This is a common question and one that can have a few different answers Should themes include plugin functionality The short answer is no The primary reason for this is because plugins are meant to add features and functionality to WordPress regardless of the theme used This creates a nice separation between your web site design and the functionality of your web site The reason this separation is needed is so your theme is not directly tied to the functionality required WordPress is built so that you can easily change your design or theme at any point with just a couple clicks If all plugin functionality existed in your theme and you switched themes you will have lost all that functionality you required There is also a strong argument that certain features should be included in a theme A common feature most themes include is breadcrumb navigation This feature could certainly exist in a plugin but being a navigation centric feature it makes sense to include this in the theme Search engine optimization features
11. hod is the best to use The first method uses the built in auto installer This method enables you to search the Plugin Directory on WordPress org directly from the admin dashboard of your WordPress web site After you find a plugin to install simply click the Install link and the plugin automatically downloads and installs The second method uses the zip uploader Zipped plugin files can be uploaded extracted and installed by WordPress To use this method click the Upload link at the top of the Install Plugins CHAPTER1 AN INTRODUCTION TO PLUGINS page Click the Browser button and select the plugin zip file you want to install After you select the plugin click the Install Now button Search Upload Featured Popular Newest Recently Updated as shown in Figure 1 3 Z Install Plugins Install a plugin in zip format The third and final method to install a plugin in WordPress uses File Transfer Protocol FTP Using FTP is simply connecting to your web server using an FTP client and manually FIGURE 1 3 uploading the plugin to your WordPress installation To use this method upload the uncompressed plugin folder or file to the wp content plugins directory on your web server if you have a plugin in a zip format you mar install it by uploading it here alloween revenge zip Browse Install Now Managing Plugins After you install a plugin in WordPress you can manage it along with all other plugins under the
12. lugin Testing Plugin Functionality On occasion you may want to test some plugin functionality without actually creating a plugin to do so Many developers will place code directly in the wo config php file to do so This is a bad technique and should not be used because when the config file is parsed and loaded WordPress is not wholly instantiated yet Instead of hacking wo config php make a test php file with the following code snippet and place it in your WordPress root directory lt php Load the WordPress Environment AAEE define WP_DEBUG true uncomment for debug mode Wrox com require wp load php require_once wp admin admin php uncomment for is_admin gt lt pre gt lt php test stuff here var_dump is_admin gt lt pre gt Code snippet test php This is a quick way to load all of the required WordPress functions to test plugin functionality without actually creating a plugin As you can see wp load php is included at the beginning of the file You can also include wp admin admin php if you want to test admin side functionality Once you have included the required WordPress core files you want test any code that would otherwise exist reside in your plugin Don t forget to remove your test php file when you are done testing SUMMARY In this chapter you learned what about plugins and how they can interact with WordPress using the available APIs The
13. lugin The primary advantage to using the Settings API is security All settings data is scrubbed so you do not need to worry about cross site request forgery CSRF and cross site scripting XSS attacks when saving plugin settings Options Stores and retrieves options in your plugin This API features the capability to create new options update existing options delete options and retrieve any option already defined Dashboard Widgets Creates admin dashboard widgets Widgets automatically appear on the Dashboard of WordPress and contain all standard customization features including minimize drag drop and screen options for hiding Rewrite Creates custom rewrite rules in your plugin This API enables you to add static end points custom page structure tags Spostname and add additional feed links feed json Transients Creates temporary options cached data in your plugins This API is similar to the Options API but all options are saved with an expiration time Database Accesses the WordPress database This includes creating updating deleting and retrieving database records for use in your plugins Available Plugins 3 WordPress also features pluggable functions These functions enable you WordPress URL Called to override specific core functions in a plugin For example the wo_mail 4 function is a pluggable function You can easily define this function in your plugin and send email usin
14. lugin Directory The first place to start when researching available WordPress plugins is the official Plugin Directory at WordPress org The Plugin Directory is located at http wordpress org extend plugins With more than 10 000 plugins available and well over 100 million plugin downloads it s easy to see the vital role plugins play in every WordPress web site All plugins available in the Plugin Directory are 100 GPL and free to use for personal or commercial use 4 CHAPTER1 AN INTRODUCTION TO PLUGINS Popular Plugin Examples Take a look at the five most downloaded WordPress plugins available to get a sense of their diversity gt All in One SEO Pack Adds advanced search engine optimization functionality to WordPress Features include custom meta data for all content canonical URLs custom post type support and more gt http wordpress org extend plugins all in one seo pack Google XML Sitemaps Generates an XML sitemap of all content for submission to the popular search engines such as Google Bing and Ask com gt http wordpress org extend plugins google sitemap generator Akismet A popular comment spam filter for WordPress Checks all comments against the Akismet web service to verify whether the comment is spam gt http wordpress org extend plugins akismet NextGEN Gallery Adds advanced image gallery support to WordPress You can easily create and manage image galleries and slideshows
15. re makes it less risky when activating and testing out new plugins Even if the plugin does cause a white screen of death error message you can easily rename the plugin folder and WordPress deactivates the plugin This makes it impossible for a rogue plugin to lock you out of your own site because of an error On the other hand if you were to hack the WordPress core you can most certainly cause fatal errors that will crash your web site This can also include making unrecoverable damage to WordPress Plugin Community A huge community is centered around plugin development sharing knowledge and code and creating wonderful plugins Getting involved in the community is a great way to take your plugin development skills to the next level Chapter 18 The Developer Toolbox covers many of these resources E Appearance INSTALLING AND MANAGING PLUGINS amp Plugins v Plugins All plugin management in WordPress happens under the Plugins screen in the Add New WordPress Dashboard as shown in Figure 1 2 Ear of Users The menu shown in Figure 1 2 is available only to administrators in WordPress T Tools so nonadministrators cannot see this menu If you use the Multisite feature of ae WordPress the Plugins menu is hidden by default You need to enable the menu under Network Admin Settings FIGURE 1 2 Installing a Plugin WordPress features three different methods for installing a new plugin Your server setup dictates which met
16. rectories A lesser known fact is WordPress actually features two plugin directories The primary plugin directory is located under wp content plugins in a standard WordPress installation The second lesser known plugin directory is located under wp content mu plugins The mu plugins Installing and Managing Plugins 9 directory which stands for Must Use is not auto created by WordPress so it must be manually created to be used The primary difference between the two is the mu plugins directory is for plugins that are always executed This means any plugin included in this directory will automatically be loaded in WordPress and across all sites in the network if you run Multi site The mu plugins directory will not read plugins in a subfolder so all plugins much be individual files or must include additional files that exist in a subdirectory Any plugin files in a subfolder will be ignored unless included in the primary plugin file Types of Plugins Wo A ES Plugins Add New WordPress features a few different types and statuses for plugins as shown in Figure 1 4 You need to understand the difference when administering and creating plugins for WordPress FIGURE 1 4 All 46 Active 22 Inactive 26 Must Use 1 Drop ins 3 Bulk Actions Apply gt Active Plugin is active and running in WordPress gt Inactive Plugin is installed but not active No code from the plugin is executed gt Must U
17. se All plugins installed in the wo content mu plugins directory All Must Use or MU plugins are loaded automatically The only way to deactivate an MU plugin is to remove it completely from the directory gt Drop ins Core functionality of WordPress can be replaced by Drop in plugins These plugins are a specifically named PHP files located in the wp content directory If WordPress detects one of these files it will be auto loaded and listed under the Drop in filter on the Plugin screen Currently ten Drop in plugins are available gt advanced cache php Advanced caching plugin db php Custom database class db error php Custom database error message install php Custom installation script maintenance php Custom maintenance message object cache php External object cache sunrise php Advanced domain mapping blog deleted php Custom blog deleted message blog inactive php Custom blog inactive message Y Y VV VV VY VY Y blog suspended php Custom blog suspended message 10 CHAPTER1 AN INTRODUCTION TO PLUGINS The last four drop in plugins are specific to the WordPress Multisite feature A standard WordPress installation will have no use for these plugins When developing a new plugin determine what type of plugin you want to create before you start the development process Most plugins will be standard WordPress plugins but occasionally you might need to create a Must Use or Drop in specific p
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