PHP Exponential Decay Calculator

I've searched the web for hours looking for any kind of PHP scripts that calculate exponential decay. I was surprised not to find any, so I've written one up quickly. It's not very advanced, but it gets the job done, and it does what I need it for. It includes a simple form to enter values, and displays the result once the form is submitted. I will supply the script here for anyone to take and use. If you modify or add to it, please supply me with the new code and I will update it here for others to use as well. The formula used is:
N(t) = N₀e^-kt
For more help on exponential decay, I found these links to be useful to me:

• http://www.youtube.com/watch?v=HTDop6eEsaA (YouTube video)
  
• http://www.wtamu.edu/academic/anns/mps/math/mathlab/col_algebra/col_alg_tut47_growth.htm

How to use - If you know the quantity for a time, this script will solve for k. Enter the initial time t, and the quantity k (for example, .5 for half-life), then enter the initial time and quantity for which you want to solve.

Source Code:

<!--
// Exponential Decay Calculator
// Original Author: Mike Burch
// Created: 29 July 2010
// Description: A simple form and script to calculate exponential decay.
-->
<h2>Exponential Decay Calculator</h2>
Formula: <b><i>N(t) = N<sub>0</sub>e<sup>-kt</sup></i></b><br>
<form action="<?php echo $_SERVER['PHP_SELF'];?>" method="POST">
<table>
<tr><td><b><i>t</i></b> (Initial Time)</td>
<td><input type="text" name="ti" value="<?php echo @$_POST['ti']; ?>"></td></tr>
<tr><td><b><i>k</i></b> (Quantity)</td>
<td><input type="text" name="k" value="<?php echo @$_POST['k']; ?>"></td></tr>
<tr><td><b><i>t</i></b> (New Time)</td>
<td><input type="text" name="t" value="<?php echo @$_POST['t']; ?>"></td></tr>
<tr><td><b><i>N<sub>0</sub></i></b> (Initial Quantity)</td>
<td><input type="text" name="N0" value="<?php echo @$_POST['N0']; ?>"></td></tr>
<tr><td colspan="2" align="right"><input type=submit name="calc" value="Calculate">
</td></tr>
</table>
</form>
<?php
if (isset($_POST['calc']))
{
$k = $_POST['k'];
$ti = $_POST['ti'];
$t = $_POST['t'];
$N0 = $_POST['N0'];
$k = log($k) / $ti;
$k = -$k * $t;
$N = pow(2.718281828, -$k) * $N0;
echo "Result: <b>$N</b>";
}
?>
<p>How to use: If after <i><b>t</b></i> (initial time) there is <i><b>k</b></i> 
quantity left, how<br>
much will be left after <i><b>t</b></i> (new time), with an initial quantity of 
<b><i>N<sub>0</sub></i></b>?</p>