keep adding value in php
<p>Initially,the $number = 0. After 1st time clicking Add button, the value becomes 10. When 2nd time clicking, the value is changed to 20. then 3rd time is 30, 4th time 40.</p>

<p>Below is my code, is there anyone know how to fix it? Thanks!</p>

<pre><code>&lt;?php
      $number = 0;
      if(isset($_POST['add'])){

          $number = $number +10;
       }
?&gt;
&lt;html&gt;
    &lt;head&gt;

    &lt;/head&gt;
    &lt;body&gt;
        &lt;form method="post" action="&lt;?php echo $_SERVER['SCRIPT_NAME']; ?&gt;"&gt;
            &lt;?php echo $number; ?&gt;
            &lt;input type="submit" name="add" value="Add" /&gt;
        &lt;/form&gt;
    &lt;/body&gt;
&lt;/html&gt;
</code></pre>
