Untitled

<!doctype html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
</head>
<body>

<?/*
$_GET['sign']='*';
$_GET['a']=1;
$_GET['b']=2';

$sign=$_GET['sign'];
$a=$_GET['a'];
$b=$_GET['b'];
*/?>
	<form name="calc" method="get" action="">
	Введите a: <input type="text" name="a" value="<?_GET['a']?>"><br/>
    Введите b: <input type="text" name="b" value="<?_GET['b']?>"><br/>

	<select name="sign">
	<?  $operations = ['+', '-', '*', '/'];
	foreach ($operations as $sign)
	{ ?>
	<option value="<? echo $sign;?>"><? echo $sign;?></option><?} ?>
	</select>
	<input type="submit" value="=">

	</form>

	<?
$sign="*";
$a=1;
$b=1;
$sign=$_GET['sign'];
$a=$_GET['a'];
$b=$_GET['b'];


//echo var_dump(!empty($a));
//echo var_dump(!empty($b));
//echo var_dump($a);
//echo var_dump($b);

if (is_numeric ($a)&&is_numeric($b)&&(!empty($a))&&(!empty($b))) {
      	   if  ($sign=='*') {echo $a*$b;}
      else if  ($sign=='-') {echo $a-$b;}
      else if  ($sign=='+') {echo $a+$b;}
      else if (($sign=='/')
      	      &&('b'!=0)) {echo $a/$b;}
      else                  {echo 'Error';}
      	}
      else {echo "Введите число";}
	?>
</body>
</html>