WordPress plugin development

The WP_Filesystem API: write files the way WordPress does When WordPress updates a plugin, it does not always write files with PHP directly. On some servers PHP runs as a user that may not change the WordPress files, and WordPress asks for FTP or SSH credentials instead. The WP_Filesystem API is the layer that hides…

Rewrite rules in WordPress: custom post type slugs, endpoints and flushing WordPress turns pretty URLs into queries with a table of rewrite rules: regular expressions that map a path like /books/my-book/ to query variables like post_type=book&name=my-book. The table is generated by the WP_Rewrite class and stored in the rewrite_rules option. Custom post types, taxonomies and…

wp_list_pluck(), wp_list_filter() and wp_list_sort(): array helpers in WordPress WordPress returns lists everywhere: get_posts() gives WP_Post objects, get_terms() gives WP_Term objects, get_users() gives WP_User objects. Getting one field from all of them, keeping some of them or sorting them by a field are everyday tasks, and core has three small functions for them that work with…

Plural forms and placeholders in WordPress translations The basic translation functions are easy: __() returns a string, _e() prints it. The difficult part is sentences with numbers and names. “3 posts deleted” built with concatenation and an if ( 1 === $count ) check is untranslatable: other languages put the number elsewhere, and many have…

Paths and URLs in WordPress: plugin_dir_path(), get_stylesheet_directory() and friends A plugin or theme needs two kinds of locations: file system paths for PHP (require, file_exists(), reading files) and URLs for the browser (stylesheets, scripts, images, links). Mixing them up, or hard-coding /wp-content/, is a typical source of bugs that appear only on another server, with…

Dates and time zones in WordPress: current_time(), wp_date() and UTC Dates are one of the easiest places to make mistakes in WordPress. Posts store two dates (post_date in the site’s time zone and post_date_gmt in UTC), WordPress sets PHP’s default time zone to UTC, and the site’s time zone from Settings → General is applied…

WP_Error: return errors the WordPress way A function that returns false on failure tells the caller that something went wrong, but not what. WordPress has its own error object for this, WP_Error: it carries an error code, a message and optional data, and it can hold several errors at once. Core functions such as wp_insert_post(),…

Add a custom sidebar to the block editor with PluginSidebar The block editor has two tabs in its sidebar: Document (status, categories, tags) and Block (settings of the selected block). A plugin can add its own sidebar next to them, with a button in the top toolbar and an entry in the “More tools” menu.…

Useful WordPress functions for themes Exactly one month ago I published an article about useful WooCommerce functions. This one is a kind of continuation, but here are only WordPress functions (and a couple for ACF) that I use in almost every project. They all go into functions.php.