Nah, PHP won't parse symbols like that, it will just print it back to the page.

Try this instead...

<?php
$form = "<form action=\"test.php\" method=\"post\"><input type=\"text\" value=\"\" name=\"one\"/><br/><input type=\"text\" value=\"\" name=\"two\"/><br/><input type=\"text\" value=\"\" name=\"three\"/><br/><input type=\"submit\" value=\"submit\" name=\"submit\"/></form>";

if($_POST["submit"])
{
  if($_POST["two"] == "+") {$value = $_POST["one"] + $_POST["three"]; echo $_POST["one"] ." + " . $_POST["three"] . " = " . $value;}
elseif($_POST["two"] == "-") {$value = $_POST["one"] - $_POST["three"]; echo $_POST["one"] . " - " . $_POST["three"] . " = " . $value;}
elseif($_POST["two"] == "*") {$value = $_POST["one"] * $_POST["three"]; echo $_POST["one"] . " * " . $_POST["three"] . " = " . $value;}
elseif($_POST["two"] == "/") {$value = $_POST["one"] / $_POST["three"]; echo $_POST["one"] . " / " . $_POST["three"] . " = " . $value;}
else echo "You have entered an unknown operator!";

}

else echo stripslashes($form);

?>