Probleem met mijn Rekenmachine in PHP

<?php
$getal1 = '

<form action="reken_uit.php" method="get">
<input type="text" name="getalvoor" onKeyPress="return numbersonly(this, event)" />
<select name="plusmin">
  <option value="+">+</option>
  <option value="2">2</option>
  <option value="3">3</option>
</select>
<input type="text" name="getalachter" onKeyPress="return numbersonly(this, event)" />
<input type="submit" />
</form> 


</form> 

';;
?>

<?=$getal1?>

<SCRIPT TYPE="text/javascript">
<!--
// copyright 1999 Idocs, Inc. http://www.idocs.com
// Distribute this script freely but keep this notice in place
function numbersonly(myfield, e, dec)
{
var key;
var keychar;

if (window.event)
   key = window.event.keyCode;
else if (e)
   key = e.which;
else
   return true;
keychar = String.fromCharCode(key);

// control keys
if ((key==null) || (key==0) || (key==8) || 
    (key==9) || (key==13) || (key==27) )
   return true;

// numbers
else if ((("0123456789").indexOf(keychar) > -1))
   return true;

// decimal point jump
else if (dec && (keychar == "."))
   {
   myfield.form.elements[dec].focus();
   return false;
   }
else
   return false;
}

//-->
</SCRIPT> 

<?php
$getal1 = '
 
<form action="reken_uit.php" method="get">
<input type="text" name="getalvoor" onKeyPress="return numbersonly(this, event)" />
<select name="plusmin">
  <option value="+">+</option>
  <option value="2">2</option>
  <option value="3">3</option>
</select>
<input type="text" name="getalachter" onKeyPress="return numbersonly(this, event)" />
<input type="submit" />
</form> 
 
 
</form> 
 
';;
?>
 
<?=$getal1?>
 
<SCRIPT TYPE="text/javascript">
<!--
// copyright 1999 Idocs, Inc. http://www.idocs.com
// Distribute this script freely but keep this notice in place
function numbersonly(myfield, e, dec)
{
var key;
var keychar;
 
if (window.event)
   key = window.event.keyCode;
else if (e)
   key = e.which;
else
   return true;
keychar = String.fromCharCode(key);
 
// control keys
if ((key==null) || (key==0) || (key==8) || 
    (key==9) || (key==13) || (key==27) )
   return true;
 
// numbers
else if ((("0123456789").indexOf(keychar) > -1))
   return true;
 
// decimal point jump
else if (dec && (keychar == "."))
   {
   myfield.form.elements[dec].focus();
   return false;
   }
else
   return false;
}
 
//-->
</SCRIPT>

 <?
$gv = $_GET["getalvoor"];
$ga = $_GET["getalachter"];
$pm = $_GET["plusmin"];

$resultaat = $gv + $ga;
echo $resultaat;
?>  

<?
$gv = $_GET["getalvoor"];
$ga = $_GET["getalachter"];
$pm = $_GET["plusmin"];
 
$resultaat = $gv + $ga;
echo $resultaat;
?>

<hier je form>
   <hier controls />
</einde form>

<?php

  //hier de PHP code voor de verwerking van je formulier

?>

<hier je form>
   <hier controls />
</einde form>
 
<?php
 
  //hier de PHP code voor de verwerking van je formulier
 
?>

switch ($pm)
{
  case "+" : $result = $getalvoor + $getalachter; break;
  case "-" : $result = $getalvoor - $getalachter; break;
  case "*" : $result = $getalvoor * $getalachter; break;
  case "/" : $result = $getalvoor / $getalachter; break;
}

echo $result;

switch ($pm)
{
  case "+" : $result = $getalvoor + $getalachter; break;
  case "-" : $result = $getalvoor - $getalachter; break;
  case "*" : $result = $getalvoor * $getalachter; break;
  case "/" : $result = $getalvoor / $getalachter; break;
}
 
echo $result;

<form action="" method="post">
<input type="text" name="veld1">
<select name="operator">
<option value="+">+</option>
</select>
<input type="text" name="veld2">
<input type="submit" name="submit" value="Bereken!">
</form>

<?php
if($_SERVER['REQUEST_METHOD'] == 'POST')
{
  if($_POST['operator'] == '+')
  {
    echo $_POST['veld1'] + $_POST['veld2'];
  }
}
?>

<form action="" method="post">
<input type="text" name="veld1">
<select name="operator">
<option value="+">+</option>
</select>
<input type="text" name="veld2">
<input type="submit" name="submit" value="Bereken!">
</form>
 
<?php
if($_SERVER['REQUEST_METHOD'] == 'POST')
{
  if($_POST['operator'] == '+')
  {
    echo $_POST['veld1'] + $_POST['veld2'];
  }
}
?>