什么时候用GET？什么时候用POST？

GET和POST两种方法都是将数据送到服务器，但你该用哪一种呢？

HTTP标准包含这两种方法是为了达到不同的目的。POST用于创建资源，资源的内容会被编入HTTP请示的内容中。例如，处理订货表单、在数据库中加入新数据行等。

当请求无副作用时（如进行搜索），便可使用GET方法；当请求有副作用时（如添加数据行），则用POST方法。一个比较实际的问题是：GET方法可能会产生很长的URL，或许会超过某些浏览器与服务器对URL长度的限制。

若符合下列任一情况，则用POST方法：

* 请求的结果有持续性的副作用，例如，数据库内添加新的数据行。
* 若使用GET方法，则表单上收集的数据可能让URL过长。
* 要传送的数据不是采用7位的ASCII编码。

若符合下列任一情况，则用GET方法：

* 请求是为了查找资源，HTML表单数据仅用来帮助搜索。
* 请求结果无持续性的副作用。
* 收集的数据及HTML表单内的输入字段名称的总长不超过1024个字符。

以上内容摘自《Web Database Application with PHP and MySQL, 2nd Edition》一书，中文版《PHP & MySQL Web数据库应用开发指南》。英文原文内容如下：
GET versus POST

Both the GET and POST methods send data to the server, but which method should you use?

The HTTP standard includes the two methods to achieve different goals. The POST method was intended to create a resource. The contents of the resource would be encoded into the body of the HTTP request. For example, an order form might be processed and a new row in a database created.

The GET method is used when a request has no side effects (such as performing a search) and the POST method is used when a request has side effects (such as adding a new row to a database). A more practical issue is that the GET method may result in long URLs, and may even exceed some browser and server limits on URL length.

Use the POST method if any of the following are true:

* The result of the request has persistent side effects such as adding a new database row.
* The data collected on the form is likely to result in a long URL if you used the GET method.
* The data to be sent is in any encoding other than seven-bit ASCII.

Use the GET method if all the following are true:

* The request is to find a resource, and HTML form data is used to help that search.
* The result of the request has no persistent side effects.
* The data collected and the input field names in a HTML form are in total less than 1,024 characters in size.