According to the HTTP specification, you should use the POST method when you're using the form to change the state of something on the server end. For example, if a page has a form to allow users to add their own comments, like this page here, the form should use POST. If you click "Reload" or "Refresh" on a page that you reached through a POST, it's almost always an error -- you shouldn't be posting the same comment twice -- which is why these pages aren't bookmarked or cached.
You should use the GET method when your form is, well, getting something off the server and not actually changing anything. For example, the form for a search engine should use GET, since searching a Web site should not be changing anything that the client might care about, and bookmarking or caching the results of a search-engine query is just as useful as bookmarking or caching a static HTML page.
处理表单
PHP 一个很有用的特点体现在它处理 PHP 表单的方式。需要理解的非常重要的原理，是表单的任何元素都在 PHP 脚本中自动生效。请参阅本手册中“PHP 的外部变量”以获取关于在 PHP 中使用表单的详细信息及范例。以下是 HTML 表单的范例：
示例 #1 一个简单的 HTML 表单
<form action="action.php" method="post">
<label for="name">Your name:</label>
<input name="name" id="name" type="text">
<label for="age">Your age:</label>
<input name="age" id="age" type="number">
<button type="submit">Submit</button>
</form>
该表单中并没有什么特殊的地方，其中没有使用任何特殊的标识符。当用户填写了该表单并点击了提交按钮，页面 action.php 将被调用。在该文件中，可以加入如下内容：
示例 #2 打印来自表单的数据
你好，<?php echo htmlspecialchars($_POST['name']); ?>。
你 <?php echo (int)$_POST['age']; ?> 岁了。该脚本的输出可能是：
你好，Joe。你 22 岁了。
除了htmlspecialchars() 和 (int)
部分，这段程序做什么用显而易见。htmlspecialchars()
使得 HTML 之中的特殊字符被正确的编码，从而不会被使用者在页面注入 HTML 标签或者
Javascript 代码。例如 age 字段，我们明确知道他是一个数值，因此我们将它转换为一个int来自动的消除任何不必要的字符。也可以使用 PHP 的 filter 扩展来自动完成该工作。PHP
将自动设置 $_POST['name'] 和
$_POST['age'] 变量。在这之前我们使用了超全局变量
$_SERVER，现在我们引入了超全局变量
$_POST，它包含了所有的
POST 数据。请注意我们的表单提交数据的方法（method）。如果使用了
GET 方法，那么表单中的信息将被储存到超全局变量
$_GET
中。如果并不关心请求数据的来源，也可以用超全局变量
$_REQUEST，它包含了所有
GET、POST、COOKIE 和 FILE 的数据。
也可以在 PHP 中处理 XForms 的输入，尽管用户可能更喜欢使用长久以来支持良好的 HTML 表单。XForms 目前还不适合初学者使用，但是用户可能对它感兴趣。手册中在“特点”一章有一节对如何处理从 XForum 接收到的数据进行了简短的介绍。
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User Contributed Notes 3 notes
sethg at ropine dot com ¶
21 years ago
Johann Gomes (johanngomes at gmail dot com) ¶
14 years ago
Also, don't ever use GET method in a form that capture passwords and other things that are meant to be hidden.
nucc1 ¶
7 years ago
worth clarifying:
POST is not more secure than GET.
The reasons for choosing GET vs POST involve various factors such as intent of the request (are you "submitting" information?), the size of the request (there are limits to how long a URL can be, and GET parameters are sent in the URL), and how easily you want the Action to be shareable -- Example, Google Searches are GET because it makes it easy to copy and share the search query with someone else simply by sharing the URL.
Security is only a consideration here due to the fact that a GET is easier to share than a POST. Example: you don't want a password to be sent by GET, because the user might share the resulting URL and inadvertently expose their password.
However, a GET and a POST are equally easy to intercept by a well-placed malicious person if you don't deploy TLS/SSL to protect the network connection itself.
All Forms sent over HTTP (usually port 80) are insecure, and today (2017), there aren't many good reasons for a public website to not be using HTTPS (which is basically HTTP + Transport Layer Security).
As a bonus, if you use TLS you minimise the risk of your users getting code (ADs) injected into your traffic that wasn't put there by you.
