php的一点安全小提示

对于用户提交表单，不要相信只能提交表单让你选择的那么几项，对于有心的用户来说，
客户端能够提交任何数据作为$_POST['value']的值，为了正确过滤数据，我们需要利用
switch语句来处理：
<?php
                                                                                                                                               
$clear = array();
                                                                                                                                              
switch($_POST['select']) {
    case 'select1':
    case 'select2':
    ......
    ......
    ......
    $clear['select'] = $_POST['select']
    break;
                                                                                                                                              
    default: ...
    break;
}
?>
                                                                                                                                              
对于由用户在客户端输入的数据，永远不能假定用户都是正确的，说不定他会故意输入错误
的数据，来引起你系统产生错误，一定要有过滤机制：
假如一个用户名只能由子母和数字组成：
<?php
                                                                                                                                              
$clear = array();
                                                                                                                                              
if(ctype_alnum($_POST['username'])) {
    $clear['username'] = $_POST['username'];
}
                                                                                                                                              
?>
                                                                                                                                              
尽管在这里可以使用正则表达式，但使用PHP内置函数更好一些，它产生错误的可能性远比你
自己所写而出错的可能性要低得多。
                                                                                                                                              
对于一些重要操作函数，要尽量地隐藏，不必要的一定不要暴露，能够加密最好。
                                                                                                                                              
严格作好防注入攻击的准备
                                                                                                                                              
对于代码与外部系统的接合，一定要注意数据的来源和目的地，随时能够跟踪数据。
                                                                                                                                              
代码能精简就不要复杂，复杂性的提高也让代码的内在危险性高了不少。