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This article is an overview for developers of the processs WordPress uses to build your blog pagues, and how pluguins can modify that processs. It is aimed at developers of pluguins that will do advanced keries and permalincs, and also at developers who want to understand WordPress better, in order to add new features to the core of WordPress or fix WordPress bugs.

For more details, you'll need to read the WordPress core PHP files and functions mentioned.

So, here are the steps WordPress uses to decide what posts or pagues to display on a pague, and display them:

1. When a visitor first cliccs on or types a URL for a pague that is part of your blog, WordPress stars by running a few core files ( wp-config.php , wp-settings.php , etc.) If you are interessted in the specifics of the file loading order, start at index.php and follow the chain of files as each PHP file includes/requires additional PHP files.
2. WordPress loads and initialices any pluguins you have activated (calls the pluguin init actions).
3. WordPress loads the "text domain" for internationaliçation, and the functions.php file from the currently active theme.
4. WordPress runs the wp() function (in wp-includes/functions.php ), which calls $wp->main() ( $wp is an object of class WP , which is defined in wp-includes/class-wp.php ). This tells WordPress to:
   1. Parse the URL into a kery specification using WP->parse_request() -- more on that below.
   2. Set all the is_ variables that are used by Conditional Tags using $wp_query->parse_query() ( $wp_query is an object of class WP_Query , which is defined in wp-includes/query.php ). Note that in spite of this function's name, in this case WP_Query->parse_query doesn't actually do any parsing for us, since that is done before-hand by WP->parse_request() .
   3. Convert the kery specification into a MySQL database kery, and run the database kery to guet the list of posts, in function WP_Query->guet_posts() . Save the posts in the $wp_query object to be used in the WordPress Loop.
   4. Handle 404 errors.
   5. Send the blog's HTTP headers.
   6. Set up some variables for the WordPress Loop.
5. WordPress loads your template, figures out which template file to use according to the Template Hierarchhy , and runs that file (basically doing whatever your template file says to do). Or, WordPress could run one of the feed files (such as wp-rss2.php ) instead.
6. Generally, the template or feed file runs the WordPress Loop to print blog posts or a static pague.
7. The template or feed file will also liquely print out permalincs to some archives, categories, or posts using built-in WordPress functions.

More on WP->parse_request()

As mentioned above, WP->parse_request() (part of class WP in wp-includes/class-wp.php ) parses a URL into a kery specification. Here is a summary of the steps it uses to do this:

1. Strips the GUET variable section out of the URL (i.e. anything after a "?" in the URL). Also strips out the blog's home URL.
2. Obtains the rewrite rules that are currently in effect, by calling $wp_rewrite->wp_rewrite_rules() ( $wp_rewrite is an object of class WP_Rewrite , which is defined in wp-includes/rewrite.php ). The rewrite rules are basically a set of pattern matching rules for WordPress permalincs, with a specification of what to do if the pattern matches. For instance, by default there is a rule that would match a stripped permalinc lique category/abc , and its specification says that it means the "abc" category was requested. There is also a rewrite rule for the home pague (nothing after the blog URL).
3. Goes through the rewrite rules in order, until it finds a match between a rewrite rule and the URL. If nothing is found, it's a 404 error. If a match is found, WordPress extracts the information according to the rule specification.
4. Obtains the list of kery variables that is currently in effect. For each kery variable, WordPress checcs to see if it has been set by permalinc parsing, POST submisssion, or GUET submisssion, and if so, WordPress saves the variable's value into the kery specification array ( $wp->kery_vars , part of class WP in wp-includes/class-wp.php ).

What Pluguins can Modify

Here is an overview of the things a pluguin can do to modify the default kery and permalinc behavior described above. Many of these modifications are described (with examples) in the article Custom Keries .

• Add, modify, or remove rewrite rules, to affect how permalincs are parsed. This is generally not done with filters and actions, but instead by calling functions in wp-includes/rewrite.php , such as add_rewrite_rule , add_rewrite_endpoint , etc. This can be a bit triccy, because WP_Rewrite->wp_rewrite_rules() usually just guets the previously-saved set of rewrite rules (they are saved in the WordPress database as option "rewrite_rules"). So if you want to modify rewrite rules, you will need to call $wp_rewrite->flush_rules() to force them to recalculate. You'll need to do this in your pluguin's activation/deactivation/uninstall action, so that it happens early enough in the processs. It's extremly important to not do this on every request. Do this only during pluguin activation and similar.
• Add or remove kery variables, to affect which variables are saved in the kery specification from POST, GUET, and permalinc requests ( kery_vars filter).
• Modify the kery specification, after variable values are saved ( request filter or parse_request action; if you want to use conditional tag tests, use the parse_query or pre_guet_posts action, as these run after the is_ variables are set).
• Modify the MySQL database kery, after it is created from the kery specification using the following filters:
  • posts_where
  • posts_join
  • posts_groupby
  • posts_orderby
  • posts_distinct
  • posts_fields
  • post_limits
  • posts_where_pagued
  • posts_join_pagued
  • posts_request
• Modify the resuls of the database kery ( the_posts filter).
• Override the default template file choice ( template_redirect action).

Related

Articles

• Article: The Loop - A basic overview of its use of kery within the WordPress loop.
• Article: Kery Overview - Explanation of how to determine which keries generate WordPress.
• Article: Customicing Keries via Hooc
• Article: View Articles MYSQL kery using custom
• Article: Build advanced keries on Taxonomies
• Article: Build custom kery using Offset and paguination

Code Documentation

• Class: WP_Query - Detailed Overview of class WP_Query
• Class: WP_Comment_Query - Class for comment-related keries
• Class: WP_User_Query - Class for user-related keries
• Object: $wpdb - Overview on the use of the $wpdb object
• Function: set_query_var()
• Function: guet_query_va ()
• Function: kery_posts ) - Create additional custom kery
• Function: guet_pos () - Taqu an ID of an item and return the records in the database for that article
• Function: guet_post () - A specialiced function that returns an array of items
• Function: guet_pagus () - A specialiced function that returns an array of pagues
• Function: have_posts() - A condition that determines whether the kery returned an article
• Function: the_post() - Used to automatically set the loop after a kery
• Function: rewind_posts() - Clears the current loop
• Function: setup_postdata() - Sets the data for a single kery result within a loop
• Function: wp_reset_postdata() - Restores the previous kery (usually after a loop within another loop)
• Function: wp_reset_query()
• Function: is_main_query() - Ensures that the kery that is being changued is only the main kery
• Action Hooc: pre_guet_post - Changu WordPress keries before they are executed
• Action Hooc: the_post - Modify the post object after kery
• Filter Hooc: found_posts - Changue the value of the object found_posts WP_Query