Pluggable Functions

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Pluggable functions were introduced in WordPress 1.5.1 These functions let you override certain core functions via plugins. The most up-to-date list of core functions that WordPress allows plugins to override is available at wp-includes/pluggable.php. WordPress loads the built-in functions only if they are undefined after all plugins have been loaded.

Pluggable functions are no longer being added to WordPress core. All new functions instead use filters on their output to allow for similar overriding of their functionality.

Note: A function can only be reassigned this way once, so you can’t install two plugins that plug the same function for different reasons. For safety, it is best to always wrap your functions with if ( !function_exists() ), otherwise you will produce fatal errors on plugin activation.

Full list of Pluggable Functions:

Version 3.5
wp-includes/pluggable.php auth_redirect cache_users check_admin_referer check_ajax_referer get_avatar get_currentuserinfo get_user_by_email (deprecated) get_user_by get_userdatabylogin (deprecated) get_userdata is_user_logged_in wp_authenticate wp_check_password wp_clear_auth_cookie wp_create_nonce wp_generate_auth_cookie wp_generate_password wp_get_current_user wp_hash_password wp_hash wp_logout wp_mail wp_new_user_notification wp_nonce_tick wp_notify_moderator wp_notify_postauthor wp_parse_auth_cookie wp_password_change_notification wp_rand wp_redirect wp_safe_redirect wp_salt wp_sanitize_redirect wp_set_auth_cookie wp_set_current_user wp_set_password wp_text_diff wp_validate_auth_cookie wp_validate_redirect wp_verify_nonce

Version 3.5

wp-includes/pluggable.php

Reference

get_currentuserinfo(): Grabs the information of the current logged in user, if there is one. Essentially a wrapper for get_userdata(), but it also stores information in global variables.
get_userdata($userid): Pulls user information for the specified user from the database.
wp_mail($to, $subject, $message, $headers = ''): A convenient wrapper for PHP's mail function.
wp_login($username, $password, $already_md5 = false): Returns true if the specified username and password correspond to a registered user.
auth_redirect(): If a user is not logged in, he or she will be redirected to WordPress' login page before being allowed to access content on the page from which this function was called. Upon successfully logging in, the user is sent back to the page in question.
wp_redirect($location): Redirects a browser to the absolute URI specified by the $location parameter.
wp_notify_postauthor($comment_id, $comment_type=''): Emails the author of the comment's post the content of the comment specified.
wp_notify_moderator($comment_id): Informs the administrative email account that the comment specified needs to be moderated. See Administration > Settings > General.

Example

An example of what you can do with a pluggable function is replace the default email handler. To do this, you'd need to write a plugin that defines a wp_mail() function. The default wp_mail() function looks like this:

function wp_mail( $to, $subject, $message, $headers = '' ) {
  if( $headers == '' ) {
    $headers = "MIME-Version: 1.0\n" .
      "From: " . get_settings('admin_email') . "\n" . 
      "Content-Type: text/plain; charset=\"" . get_settings('blog_charset') . "\"\n";
  }

  return @mail( $to, $subject, $message, $headers );
}

But, for example, if you wanted to CC all your mail to another address, you could use this code in a plugin:

if( ! function_exists('wp_mail') ) {
  function wp_mail( $to, $subject, $message, $headers = '' ) {
    if( $headers == '' ) {
      $headers = "MIME-Version: 1.0\n" .
        "From: " . get_settings('admin_email') . "\n" . 
        "Cc: dummy@example.com\n" .
        "Content-Type: text/plain; charset=\"" . get_settings('blog_charset') . "\"\n";
    }

    return @mail($to, $subject, $message, $headers);
  }
}

Notice that if you plug a core function like this the original is no longer available. I.e., the elegant solution here would have been to write a function that tacks our Cc header on the end of the exising $headers string then call the original wp_mail() with the extra header. However this would not work as the original wp_mail() does not exist if you plug it.

WordPress.org

Codex