I have a problem with a php calculator application. I wanted to make something like this: 

When the results appear and I hit the reset button, the reset function should not apply just to HTML, but to the result echo, too. 

I don't want the page to be refreshed. 

Here is some code:

```
<form method="POST">

<input type="number" name="fn"></input>
<select name="operation">
<option value="p">+</option>
<option value="mi">-</option>
<option value="mu">*</option>
<option value="d">/</option>
</select>
<input type="number" name="sn"></input> <br/>
<input type="submit" name="sub"></input>
<input type="reset" name="restart"></input>

</form>

<?php 
if (isset($_REQUEST['sub'])) {

if($_REQUEST["operation"] == "p") {
$fn = $_REQUEST["fn"];
$sn = $_REQUEST["sn"];

$res = $fn + $sn;

ob_start();
$restring = "The result of " . $fn . " + " . $sn . " is: " . "<b>" . $res . "</b>";
echo $restring;

if(isset($_REQUEST["restart"])) {
global $restring;
ob_end_clear();
}
}

if($_REQUEST["operation"] == "mi") {
$fn = $_REQUEST["fn"];
$sn = $_REQUEST["sn"];

$res = $fn - $sn;

echo "The result of " . $fn . " - " . $sn . " is: " . "<b>" . $res . "</b>";

}

if($_REQUEST["operation"] == "mu") {
$fn = $_REQUEST["fn"];
$sn = $_REQUEST["sn"];

$res = $fn * $sn;

echo "The result of " . $fn . " * " . $sn . " is: " . "<b>" . $res . "</b>";

}

if($_REQUEST["operation"] == "d") {
$fn = $_REQUEST["fn"];
$sn = $_REQUEST["sn"];

$res = $fn / $sn;

echo "The result of " . $fn . " / " . $sn . " is: " . "<b>" . $res . "</b>";

}

}
?>

```