Source

File: wp-includes/functions.php

function wp_get_raw_referer() {
	if ( ! empty( $_REQUEST['_wp_http_referer'] ) ) {
		return wp_unslash( $_REQUEST['_wp_http_referer'] );
	} elseif ( ! empty( $_SERVER['HTTP_REFERER'] ) ) {
		return wp_unslash( $_SERVER['HTTP_REFERER'] );
	}

	return false;
}