Source

File: wp-includes/general-template.php

function wp_register( $before = '<li>', $after = '</li>', $echo = true ) {
	if ( ! is_user_logged_in() ) {
		if ( get_option( 'users_can_register' ) ) {
			$link = $before . '<a href="' . esc_url( wp_registration_url() ) . '">' . __( 'Register' ) . '</a>' . $after;
		} else {
			$link = '';
		}
	} elseif ( current_user_can( 'read' ) ) {
		$link = $before . '<a href="' . admin_url() . '">' . __( 'Site Admin' ) . '</a>' . $after;
	} else {
		$link = '';
	}

	/**
	 * Filters the HTML link to the Registration or Admin page.
	 *
	 * Users are sent to the admin page if logged-in, or the registration page
	 * if enabled and logged-out.
	 *
	 * @since 1.5.0
	 *
	 * @param string $link The HTML code for the link to the Registration or Admin page.
	 */
	$link = apply_filters( 'register', $link );

	if ( $echo ) {
		echo $link;
	} else {
		return $link;
	}
}