Por que no funciona este codigo php
<!DOCTYPE html>
<html>
<head>
<title>Calculadora</title>
<style>
.box
{
border:1px solid green;
padding: 5px;
width: 140px;
height: 130px;
}
.box2
{
border:1px solid green;
padding: 5px;
width: 200px;
height: 20px;
}
.size
{
font-size: 20px;
}
</style>
</head>
<body>
<form name="form1" method="post">
<div class="box2">
<?php
?>
</div>
 
<div class="box">
<p>
<input type="submit" name="1" id="b1" value="1">
<input type="submit" name="2" id="b2" value="2">
<input type="submit" name="3" id="b3" value="3">
 
<input type="submit" name="sumar" id="sumar" value="+">
<input type="submit" name="multiplicar" id="multiplicar" value="*">
</p>
<p>
<input type="submit" name="4" id="b4" value="4">
<input type="submit" name="5" id="b5" value="5">
<input type="submit" name="6" id="b6" value="6">
 
<input type="submit" name="restar" id="restar" value="-">
<input type="submit" name="dividir" id="dividir" value="/">
</p>
<p>
<input type="submit" name="7" id="b7" value="7">
<input type="submit" name="8" id="b8" value="8">
<input type="submit" name="9" id="b9" value="9">
 
<input type="submit" name="igual" id="igual" value="=">
</p>
</div>
</form>
</body>
</html>
<?php
$palabra="hola";
if(isset($_POST["igual"]))
{
global $palabra;
$palabra=$palabra . "=";
}
if(isset($_POST["1"]))
{
global $palabra;
$palabra=$palabra . "1";
}
if(isset($_POST["2"]))
{
global $palabra;
$palabra=$palabra . "2";
}
if(isset($_POST["3"]))
{
global $palabra;
$palabra=$palabra . "3";
}
if(isset($_POST["4"]))
{
global $palabra;
$palabra=$palabra . "4";
}
if(isset($_POST["5"]))
{
global $palabra;
$palabra=$palabra . "5";
}
if(isset($_POST["6"]))
{
global $palabra;
$palabra=$palabra . "6";
}
if(isset($_POST["7"]))
{
global $palabra;
$palabra=$palabra . "7";
}
if(isset($_POST["8"]))
{
global $palabra;
$palabra=$palabra . "8";
}
if(isset($_POST["9"]))
{
global $palabra;
$palabra=$palabra . "9";
}
if(isset($_POST["multiplicar"]))
{
global $palabra;
$palabra=$palabra . "*";
}
if(isset($_POST["sumar"]))
{
global $palabra;
$palabra=$palabra . "+";
}
if(isset($_POST["dividir"]))
{
global $palabra;
$palabra=$palabra . "/";
}
if(isset($_POST["restar"]))
{
global $palabra;
$palabra=$palabra . "-";
}
echo $palabra;
?>
