WordPress at a glance
function is not described

trackback_url() WP 0.71

Display the current post's trackback URL.

No Hooks.

Return

null/String. Should only be used to echo the trackback URL, use get_trackback_url() for the result instead.

Usage

trackback_url( $deprecated_echo );
$deprecated_echo(true/false)
Not used.
Default: true

Code of trackback url: wp-includes/comment-template.php VER 5.0.3

<?php
function trackback_url( $deprecated_echo = true ) {
	if ( true !== $deprecated_echo ) {
		_deprecated_argument( __FUNCTION__, '2.5.0',
			/* translators: %s: get_trackback_url() */
			sprintf( __( 'Use %s instead if you do not want the value echoed.' ),
				'<code>get_trackback_url()</code>'
			)
		);
	}

	if ( $deprecated_echo ) {
		echo get_trackback_url();
	} else {
		return get_trackback_url();
	}
}