Php MySql未知错误

嗨，这里我有上面这个脚本的问题…它做的事情，但返回错误…它使更改数据库正确，但返回错误…我是不是弄错了?

$result = mysql_query(
    "INSERT INTO student (fullname, username, password, group_id) 
     VALUES ('$fullname', '$username', '$password', '$group_id')"
);
if ($result == 'true'){
    $caturi =  $group_id;
    if (isset($_POST['subject'])) {
        foreach ($_POST['subject'] as $cat) {
            $values[]  = sprintf (
                "(%d, '%s')", 
                intval($cat), 
                mysql_real_escape_string($caturi)
            );
        }
        $result2 = "INSERT INTO list (subject_a, group_a) 
                    VALUES " . join(',', $values);
        mysql_query($result2);
    }   
    echo '<p align="center" style="color:#ccc;">Success...</p>';
}
else{
    echo "Error...";
}