按钮删除以前的输出

ok, I decided to try to search this before I asked this question, but no luck.

How can I keep output from getting removed when I click on a different button? It will only display output of one or the other. Where it displays the output is correct (underneath the button) I think whats causing this is when I click the second button it goes back to its original location (where the first output is) and I think that is why I can't display both outputs at the same time.

<!DOCTYPE html>
<html>
<body>

<form method="post" action="">

<p>
<input type="submit" value="Get Uptime" name="uptimebutton" >
<?php

if(isset($_POST['uptimebutton']))
{

        //echo shell_exec('uptime');
        $uptime = shell_exec('uptime');
        echo "<pre>$uptime</pre>";                                                                


}
?></p>

<p><input type="submit" value="GRAB HDD INFO" name="memorybutton" >
<?php
if(isset($_POST['memorybutton']))
{
        $output = shell_exec('df -h');
        echo "<pre>$output</pre>";                                                 
}
?></p>
</form>
</body>
</html>

You can store them in session, or any other globally available variables. Something like this might help if you already have started a session:

<!DOCTYPE html>
<html>
    <body>
        <form method="post" action="">
            <p>
                <input type="submit" value="Get Uptime" name="uptimebutton"/>
                <?php
                    if(isset($_POST['uptimebutton']))
                    {
                        $_SESSION['uptime'] = shell_exec('uptime');
                    }
                    echo "<pre>" . (isset($_SESSION['uptime']) ? $_SESSION['uptime'] : "") . "</pre>";
                ?>
            </p>
            <p>
                <input type="submit" value="GRAB HDD INFO" name="memorybutton"/>
                <?php
                    if(isset($_POST['memorybutton']))
                    {
                        $_SESSION['output'] = shell_exec('df -h');
                    }
                    echo "<pre>" . (isset($_SESSION['output']) ? $_SESSION['output'] : "") . "</pre>";
                ?>
            </p>
        </form>
    </body>
</html>