PHP input field instant update after typing in values - php

Question

just trying to make a little calculator for myself in php and i wonder if its possible to make the answer instant update after i type in the values?
Thank you.
<?php
if (isset($_POST['valuea'])) $valuea = $_POST['valuea'];
if (isset($_POST['valueb'])) $valueb = $_POST['valueb'];
if (isset($_POST['check1'])) {
$answer = (($valuea * $valueb) * 10)*2;
} else {
$answer = ($valuea * $valueb) * 10;
}
echo <<<_END
<form method='post' action=''>
<table border='0' width='500px' cellpadding='3' cellspacing='1' class="table">
<tr class="calcrow">
<td>Lenght:</td>
<td align="center"><input type='text' name='valuea' value="$valuea"/></td>
</tr>
<tr class="calcrow2">
<td>Width:</td>
<td align="center"><input type='text' name='valueb' value="$valueb"/></td>
</tr>
<tr class="calcrow2">
<td>2x test</td>
<td align="center"><input type="checkbox" name="check1"></td>
</tr>
<tr class="submit"><td colspan="2"><input type='submit' value='Calculate'/></td></tr>
_END;
?>
<tr class="calcrow">
<td><i>Answer</td>
<td align="center"><input type="text" value="<?php echo round($answer)?>"></td></i>
</tr>
</table>
</form>

Answer 1

You are looking for a thing called Jquery.
Which can fetch current Values of your form inputs or any other elements from your webpage. Then you can then send that data to your php file where you can perform the calculation and return the result which can again be displayed wherever you want .
All of this happens in real time.
This is the way you could do it if you wanted to do it in php
Else you could always go for plain JavaScript or a mixture of jquery + javascript, to avoid using php and ajax.