Fibonacci for web developers

fib.php

<?php
    include "config.php";

    $n = $_GET['n'];

    if ($n == 1) {
        echo "fib($n) == 1"; 
        mysql_query("REPLACE INTO `fib` (`n`, `fib`) VALUES ('1','1')");
    }   
    else if ($n == 2) {
        echo "fib($n) == 1"; 
        mysql_query("REPLACE INTO `fib` (`n`, `fib`) VALUES ('2','1')");
    }   
    else {
        $i = 3;
        while ($i <= $n) {
            $res = mysql_query("SELECT `fib` FROM `fib` WHERE `n` = '$i'");

            if ($res) {
                $row = mysql_fetch_assoc($res);
                    
                if (!$row['fib']) {
                    $a = $i - 1;
                    $res = mysql_query("SELECT `fib` FROM `fib` WHERE `n` = '$a'");
                    if ($res) { 
                        $row = mysql_fetch_assoc($res);
                        $f1 = $row['fib'];
                    }   

                    $b = $i - 2;
                    $res = mysql_query("SELECT `fib` FROM `fib` WHERE `n` = '$b'");
                    if ($res) {
                        $row = mysql_fetch_assoc($res);
                        $f2 = $row['fib'];
                    }   

                    $fib = $f1+$f2;
                    mysql_query("REPLACE INTO `fib` (`n`, `fib`) VALUES ('$i','$fib')");
                }   
            }   

            $i++;
        }   

        $res = mysql_query("SELECT `fib` FROM `fib` WHERE `n` = '$n'");
        if ($res) {
            $row = mysql_fetch_assoc($res);
            echo "fib($n) == " . $row['fib'];
        }   
    }   
?>