Debug XMLRPC requests received by WordPress by logging request to a log file

xmlrpc-debug.php

<?php

// -- For debugging enable XML Logging for specfic site_id

function ws_debug_xml_post() {

  // Only capture XMLRPC requests.
	if (XMLRPC_REQUEST) {

		$logline  = "XML POST: ".date('Y-m-d H:i:s')." ".str_repeat('-', 30)."\n";
		$logline .= file_get_contents("php://input");
		$logline .= str_repeat('-', 30)."\n";

    // Specify logfile location.
		$logfile = ABSPATH.'/wp-content/xmlrpc-debug.log';

    // Dump data to file.
		@file_put_contents($logfile, $logline, FILE_APPEND | LOCK_EX);

	}

}

add_action('init', 'ws_debug_xml_post');