sqli challenge

<?php
$link = mysql_connect('localhost', 'root', '');

if (!$link) {
    die('Could not connect: ' . mysql_error());
}

if (!mysql_select_db('test')) {
    die('Could not select database: ' . mysql_error());
}

var_dump($_REQUEST);
$safe = True;

if (isset($_GET['id'])) {
    # If id is not numeric than it probably isn't safe.
   if (!is_numeric($_GET['id'])) {
        $safe = False;
    }

    # Test the value of id with a "safe" query to determine if SQLi or not.
   $query = mysql_query("SELECT 1 FROM users WHERE 1=" . $_GET['id']);

    # If query doesn't run than there is most likely no risk of SQLi.
   if (!$query)
        $safe = True;

    # Value of id is not safe, possible SQLi.
   if (!$safe)
        die("SQL INJECTION DETECTED");

        # Our "safe" query did not execute, should be safe to run on real query.
   $query = mysql_query("SELECT username FROM users WHERE id=" . $_GET['id']);

    if (!$query) {
        die('Could not query:' . mysql_error());
    }

    echo '<pre>';
    print_r(mysql_fetch_row($query));
    echo '</pre>';
} else
    echo "Did not find a value for id.";

?>