class MATH_REPLACE{
		function MATH_REPLACE(){
		}

		function replace($InputString){
			$Matchs = preg_match_all("/\(+[0-9+\s+\-.*\/()%]+\)/", $InputString, $mMatchs);
			for ($i=0;$i<$Matchs; $i++){
				$Formula = str_replace(" ", "", $mMatchs[0][$i]);
				$Formula = preg_replace("/([+-])([0-9]+)(%)/", "*(1\$1.\$2)", $Formula);
				$Formula = preg_replace("/([0-9]+)(%)/",".\$1",$Formula);
				eval("\$Result=" . $Formula . ";");
				$Pos = strpos($InputString, $mMatchs[0][$i]);
				$InputString = substr_replace($InputString, $Result, $Pos, strlen($mMatchs[0][$i]));
			}
			return $InputString;
		}
	}

	//The math expression must to be between '(' and ')' 
    $Porcent = 10;
    $Cadena = "The result of [[40+15]*2] + $Porcent% IS (((40+15)*2)+ $Porcent%)";
	$ObjSalida = new MATH_REPLACE;
	$Salida = $ObjSalida->replace($Cadena);

    echo "<BR>".$Salida;