Source

File: wp-includes/general-template.php

function wp_loginout( $redirect = '', $echo = true ) {
	if ( ! is_user_logged_in() ) {
		$link = '<a href="' . esc_url( wp_login_url( $redirect ) ) . '">' . __( 'Log in' ) . '</a>';
	} else {
		$link = '<a href="' . esc_url( wp_logout_url( $redirect ) ) . '">' . __( 'Log out' ) . '</a>';
	}

	if ( $echo ) {
		/**
		 * Filters the HTML output for the Log In/Log Out link.
		 *
		 * @since 1.5.0
		 *
		 * @param string $link The HTML link content.
		 */
		echo apply_filters( 'loginout', $link );
	} else {
		/** This filter is documented in wp-includes/general-template.php */
		return apply_filters( 'loginout', $link );
	}
}