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The wp_head action hook is triggered within the <head></head> section of the user's template by the wp_head() function. Although this is theme-dependent, it is one of the most essential theme hooks, so it is widely supported.
This hook provides no parameters. You use this hook by having your function echo output to the browser, or by having it perform background tasks. Your functions shouldn't return, and shouldn't take any parameters.
This hook is theme-dependent which means that it is up to the author of each WordPress theme to include it. It may not be available on all themes, so you should take this into account when using it.
This hook is an action which means that it primarily acts as an event trigger, instead of a content filter. This is a semantic difference, but it will help you to remember what this hook does if you use it like this:
<?php add_action('wp_head', 'your_function'); ?>
See Plugin API - Actions for more detail.
Most of default actions into the 'wp-head' hook by WordPress core are set up in
wp-includes/default-filters.php. If you need to remove a default hook, this file will give you the priority for which to use to remove the hook.
wp_head action hook is located in