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<?php$action = $_GET['do'];$num1 = $_GET['1'];$num2 = $_GET['2'];if (!isset ($_GET['1']) || !isset ($_GET['2']) || !isset ($_GET['do'])){?>You didn't enter all options<br />Usage: calc.php?do=ADD_SUBTRACT_MULTIPLY_DIVIDE&1=FIRST_NUMBER&2=SECOND_NUMBER<?php}else{switch ($action){	case 'add' : add($num1, $num2);	break;		case 'subtract' : subtract($num1, $num2);	break;		case 'multiply' : multiply($num1, $num2);	break;		case 'divide' : divide($num1, $num2);	break;	default: unknown();	break;}function add($x, $y){	echo 'Answer is: ' . $x + $y;}function subtract($x, $y){	echo 'Answer is: ' . $x - $y;}function multiply($x, $y){	echo 'Answer is: ' . $x * $y;}function divide($x, $y){	echo 'Answer is: ' . $x / $y;}function unknown($x, $y){	echo 'Unknown Option';}}?>