Also, it looks like you just dont understand server side scripting...
you left out one or two } for your loop
<BODY>
<?
if(!$number) $number = rand(1,15);
if($number!=$Guess){
if ($number>$Guess) echo "guess a bigger number";
if ($number<$Guess) echo "guess a smaller number";
?>
<FORM METHOD=POST>
<INPUT NAME="Guess" TYPE="TEXT">
<INPUT TYPE="SUBMIT">
<INPUT TYPE=HIDDEN NAME=number value="<? "echo $number"; ?>">
</FORM>
</INPUT>
<?
} else {
echo "Congragulations";
}
?>
</BODY>
This should do better.