<?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;

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