Source

File: wp-includes/functions.php

function wp_referer_field( $echo = true ) {
	$referer_field = '<input type="hidden" name="_wp_http_referer" value="' . esc_attr( wp_unslash( $_SERVER['REQUEST_URI'] ) ) . '" />';

	if ( $echo ) {
		echo $referer_field;
	}

	return $referer_field;
}