WordPress Functions
WordPress has a relevant number of functions which make easy for plugin developers and theme designers to write their scripts.
The best pages to check for the WordPress functions are, respectively, the official Function Reference and the Template Tags Page.
The Function Reference Page is structured in columns, and lists the functions per the following categories:
Post, Page, Attachment and Bookmarks Functions;
Category, Tag and Taxonomy Functions;
User and Author Functions;
Feed Functions;
Comment, Ping, and Trackback Functions;
Action, Filter, and Plugin Functions;
Theme-Related Functions;
Formatting Functions;
Miscellaneous Functions;
Geo tags;
Classes.
Classes were created by single WordPress developers, and are found in files capabilities.php, classes.php, class-snoopy.php, locale.php, query.php, rewrite.php, script-loader.php, wp-db.php files.
On the Planethoz site it’ possible to find the first WordPress version for each function.
The Template Tags page contains all the most important references for theme designers, including inside- and outside-the-loop functions.