do_action( 'init' )

More Information

Examples:

Use init to act on $_POST data:

add_action( 'init', 'process_post' );

function process_post() {
     if( isset( $_POST['unique_hidden_field'] ) ) {
          // process $_POST data here
     }
}

Notes:

init is useful for intercepting $_GET or $_POST triggers.

load_plugin_textdomain calls should be made during init, otherwise users cannot hook into it.

If you wish to plug an action once WP is loaded, use the wp_loaded hook.