simplexml_load_string(): input conversion failed due to input error เกิดจากอะไรและมีวิธีแก้ยังไงครับ

if(isset($_POST['SmsMt']))
	{
			   $xml = $_POST['SmsMt'];	
               $date = date("Y-m-d  H : i : s");
		       $type = "type : request";
		       $toLog = $date." | ".$type." | "."\t".$xml."\n\n\n";
		       $y = date("Y");
		 	   $m = date("m");
		       $d = date("d");
			   $time = $y."_".$m."_".$d;
		       $fp = fopen("Api_log_".$time.".txt",'a+');
		       fwrite($fp,$toLog);
		       fclose($fp); 
			   
				 $xml = simplexml_load_string($xml);