<?php

$str = "SI-#CL#¤1801 BARBER LANE#CL#MILPITAS CA 95035#CL#FONE 1-408-943-0600#CL#FAX 1-408-943-0484#CL#¤CANCEL BY 5 PM - RQST-NO FEATHER"; // => CANCEL BY 5 PM - RQST-NO FEATHER
//$str = "#CL#¤1801 BARBER LANE#CL#MILPITAS CA 95035#CL#FONE 1-408-943-0600#CL#FAX 1-408-943-0484#CL#¤CANCEL BY 5 PM - RQST-NO FEATHER"; 
$res = $str;
if (strrpos($str, "SI", -strlen($str)) !== false) { // starts with SI
	$arr = explode("#¤", $str);
	if (!empty($arr)){
		$res  = array_pop($arr);
	}
}
echo $res;