cant get session to save number PHP

cant get session to save number PHP

im fairly new to PHP but I am trying to make a guessing game. the idea is
you guess the number between 1-100 and the code is meant to have a
randomly assigned code to guess until guessed, currently just working on
the assigning the number. so it doesnt randomize everytime I attempt to
guess.
<?php
session_start();
$_SESSION[var_answer] = rand (1, 100);
$var_number = $_REQUEST['number'];
if($var_number == $_SESSION[var_answer]){
echo " That is correct ;)";
}
else if($var_number <0 || $var_number>100){
echo " Sorry your guess has to be between 0 and 100";
}else if($var_number==" "||$var_number==""){
echo " Please take a guess first!";
}else if($var_number < $_SESSION[var_answer]){
echo " Smaller than mine";
}else if($var_number > $_SESSION[var_answer]){
echo " Larger than mine";
}
echo '<br/><a href="RNGG.php"> return</a>';
?>
so far the number is randomizing but as I said isnt saving per session.