<?
if(isset($_POST[s1]))
{
$content = "content 1";
}
elseif(isset($_POST[s2]))
{
$content = "content 2";
}
elseif(isset($_POST[s3]))
{
$content = "content 3";
}
else
{
$content = "default content";
}

?>

<form method=post>
<table>
<tr>

<td colspan=3>
<?=$content?>
</td>

</tr>

<tr>

<td><input type=submit name=s1 value="Value 1"></td>
<td><input type=submit name=s2 value="Value 2"></td>
<td><input type=submit name=s3 value="Value 3"></td>

</tr>

</table>
</form>