WordPress: Simple registration function for custom front-end registration forms.

WordPress Registration

/**
 * mm_register function.
 * Register a new user.
 * @access public
 * @return User errors or user is logged in.
 */
function mm_register() {

	if(!is_user_logged_in()) {
		if(!empty($_POST)) {

			// verify nonce
			$nonce = $_POST['register_nonce'];
			if ( ! wp_verify_nonce( $nonce, 'register_nonce'  ) )
			{
				die();
			}

			global $notice_key; // global notice key for setting notice messages
			$userdata = array();
			$userdata['user_email']  = stripslashes($_POST['register_email']);
			$userdata['user_login']  = stripslashes($_POST['register_username']);
			$userdata['user_pass']   = stripslashes($_POST['register_password']);

			// insert the user
			$user_id = wp_insert_user( $userdata );

			if ( is_wp_error($user_id) ) {
				// set error msg notice
				if(isset($user_id->errors['empty_user_login'])) {
					$notice_key = 'empty_user_login';
				} elseif(isset($user_id->errors['existing_user_login'])) {
					$notice_key = 'existing_user_login';
				} else {
					$notice_key = 'insert_user_error';
				}
				return $user_id->errors;
			} else {

				// log user in
				$credentials = array();
				$credentials['user_login'] = stripslashes($_POST['register_username']);
				$credentials['user_password'] = stripslashes($_POST['register_password']);
				$credentials['remember'] = false;

				$user = wp_signon( $credentials, false );
				// prepare response
				if ( is_wp_error($user) ) {
					// set error msg notice
					if(isset($user->errors['invalid_username'])) {
						$notice_key = 'invalid_username';
					} elseif(isset($user->errors['incorrect_password'])) {
						$notice_key = 'incorrect_password';
					} else {
						$notice_key = 'signon_error';
					}
					return $user->errors;
				} else {
					wp_redirect( home_url() );
					exit();
				}
				return false;
			}
		}
		return false;
	} else {
		// user is logged in already
		wp_redirect( home_url() );
		exit();
	}
	return false;
}