PHP，如果不是语句 [英] PHP if not statements

问题描述

这可能是设置服务器的方式，但是我把头撞在墙上.我想说的是，如果$action没有值或具有非"add"或"delete"的值，则出现错误，否则继续运行脚本.但是，无论$action是什么，我都会出错.

This may be the way my server is set up, but I'm banging my head against the wall. I'm trying to say that if $action has no value or has a value that is not "add" or "delete" then have an error, else keep running the script. However, I get an error no matter what $action is.

 $action = $_GET['a'];
 if((!isset($action)) || ($action != "add" || $action != "delete")){
     //header("location:index.php");
     echo "error <br>";
 }

$action的设置正确，如果运行类似if($action =="add")的命令，它将起作用.这是在我的本地主机上，因此可能是设置问题.

$action is being set properly and if run something like if($action =="add") it works. This is on my local host, so it could be a settings issue.

推荐答案

您的逻辑略有偏离.第二个||应该是&&:

Your logic is slightly off. The second || should be &&:

if ((!isset($action)) || ($action != "add" && $action != "delete"))

您可以通过尝试一个样本值来查看为什么原始行会失败.假设$action是"delete".条件逐步降低的方法如下:

You can see why your original line fails by trying out a sample value. Let's say $action is "delete". Here's how the condition reduces down step by step:

// $action == "delete"
if ((!isset($action)) || ($action != "add" || $action != "delete"))
if ((!true) || ($action != "add" || $action != "delete"))
if (false || ($action != "add" || $action != "delete"))
if ($action != "add" || $action != "delete")
if (true || $action != "delete")
if (true || false)
if (true)

糟糕！条件刚刚成功并显示错误"，但是应该失败了.实际上，如果考虑一下，无论$action的值是多少，两个!=测试之一将返回true.将||切换为&&，然后倒数第二行变为if (true && false)，该行会适当地减少为if (false).

Oops! The condition just succeeded and printed "error", but it was supposed to fail. In fact, if you think about it, no matter what the value of $action is, one of the two != tests will return true. Switch the || to && and then the second to last line becomes if (true && false), which properly reduces to if (false).

顺便说一句，有一种方法可以使用||并进行测试.您必须使用迪摩根定律(即:

There is a way to use || and have the test work, by the way. You have to negate everything else using De Morgan's law, i.e.:

if ((!isset($action)) || !($action == "add" || $action == "delete"))

您可以用英语将其读为如果没有行动(添加或移除)，那么".

You can read that in English as "if action is not (either add or remove), then".

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