<form method="post" action="calc.php" name="caclulator">
<table border="0" cellpadding="2" cellspacing ="0">
<tr>
<td colspan="4" align="center">
<h3>Simple Math Calculator</h3>
<tr><td>
<input type="text" name="form1" />
<td>
<select name="func">
<option value="add">+</option>
<option value="sub">-</option>
<option value="mult">x</option>
<option value="div">/</option>
</select>
<td>
<input type="text" name="form2">
<td>
<input type="submit" name="submit" value="submit">
</table>
</form>

<?php
$num1 = $_POST['form1'];
$num2 = $_POST['form2'];
$function = $_POST['func'];
$submit = $_POST['submit'];
?>

<?php
// Set the intergers to 0. This means that if $num1 or $num1 is text, it won't work.
$num1 = 0;
$num2 = 0;

// Making the vars of the _POST data to save time
$num1 = $_POST['form1'];
$num2 = $_POST['form2'];
$func = $_POST['func'];
$submit = $_POST['submit'];

// Checks if $num1 or $num2 is empty
if(!$num1 || !$num2){
// If it is, include form.php
include('form.php');
} else if($func == add){
// If func == add, include the form page, AND, the solution.
include('form.php');
$num3 = $num1 + $num2;
echo "<br /><br />";
echo "<b>Solution</b>: <br />";
echo "$num1 + $num2 = $num3";

} else if($func == sub){
include('form.php');
$num3 = $num1 - $num2;
echo "<br /><br />";
echo "<b>Solution</b>: <br />";
echo "$num1 - $num2 = $num3";

} else if($func == div){
include('form.php');
// The @ Sign supresses the error, because in PHP, it gives an error that you can't divide by 0
@ $num3 = $num1 / $num2;
// This then checks if $num2 = 0
if ($num2 == 0){
// Tells user that you can't divide it
echo '<b>You can not divide a number by 0</b>';
// Exits the script, so it can't echo it or anything
exit;
}
echo "<br /><br />";
echo "<b>Solution</b>: <br />";
echo "$num1 / $num2 = $num3";

}else{
include('form.php');
$num3 = $num1 * $num2;
echo "<br /><br />";
echo "<b>Solution</b>: <br />";
echo "$num1 * $num2 = $num3";
}
?>