PHP if not 语句
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PHP if not statements
提问by Brooke.
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 $actionhas 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 $actionis.
这可能是我的服务器的设置方式，但我正在用头撞墙。我想说的是，如果$action没有值或值不是“添加”或“删除”，则出现错误，否则继续运行脚本。但是，无论如何我都会收到错误$action。
$action = $_GET['a'];
if((!isset($action)) || ($action != "add" || $action != "delete")){
//header("location:index.php");
echo "error <br>";
}
$actionis being set properly and if run something like if($action =="add")it works. This is on my local host, so it could be a settings issue.
$action设置正确，如果运行类似的东西if($action =="add")。这是在我的本地主机上，因此可能是设置问题。
回答by John Kugelman
Your logic is slightly off. The second ||should be &&:
你的逻辑有点不对劲。第二个||应该是&&：
if ((!isset($action)) || ($action != "add" && $action != "delete"))
You can see why your original line fails by trying out a sample value. Let's say $actionis "delete". Here's how the condition reduces down step by step:
您可以通过尝试样本值来了解原始行失败的原因。让我们说$action是"delete"。以下是条件如何逐步降低：
// $action == "delete"
if ((!isset($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)
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 $actionis, one of the two !=tests will return true. Switch the ||to &&and then the second to last line becomes if (true && false), which properly reduces to if (false).
哎呀！条件刚刚成功并打印“错误”，但它应该失败。其实仔细想想，不管是什么值，$action两个!=测试中的一个都会返回true。切换||到&&，然后倒数第二行变为if (true && false)，适当地减少到if (false)。
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.:
顺便说一下，有一种方法可以使用||并进行测试。您必须使用德摩根定律否定其他所有内容，即：
if ((!isset($action)) || !($action == "add" || $action == "delete"))
You can read that in English as "if action is not (either add or remove), then".
您可以用英语将其读为“如果操作不是（添加或删除），则”。
回答by Jeremy Ruten
No matter what $action is, it will always either not be "add" OR not be "delete", which is why the if condition always passes. What you want is to use && instead of ||:
不管 $action 是什么，它总是不是“add”或不是“delete”，这就是 if 条件总是通过的原因。你想要的是使用 && 而不是 || ：
(!isset($action)) || ($action !="add" && $action !="delete"))
回答by fastcodejava
You could also try:
你也可以试试：
if ((!isset($action)) || !($action == "add" || $action == "delete")) {
// Do your stuff
}
回答by Artefacto
You're saying "if it's not set or it's different from add or it's different from delete". You realize that a != x && a != y, with x != yis necessarily falsesince acannot be simultaneously two different values.
你是说“如果它没有设置或者它不同于添加或者它不同于删除”。您意识到a != x && a != y, withx != y是必然的，false因为a不能同时是两个不同的值。
回答by ina
For future reference, you can quickly create a truth table to check if it evaluates the way you want... it's kind of like Sudoku.
为了将来参考，您可以快速创建一个真值表来检查它是否以您想要的方式评估......它有点像数独。
(!isset($action)) && ($action != "add" && $action != "delete"))
Example:
例子：
column 1 is issetaction, column 2 and 3 evaluates !="add","delete" respectively
column 1 is issetaction, column 2 and 3 evaluates !="add","delete" respectively
if($a=add) T && (F && T) => T && F => FALSE
if($a=delete) T && (T && F) => T && F => FALSE
if($a=add) T && (F && T) => T && F => FALSE
if($a=delete) T && (T && F) => T && F => FALSE
if($a=nothing) T && (T && T) => T && T => TRUE
if($a=nothing) T && (T && T) => T && T => TRUE
回答by Amit Sharma
I think this is the best and easiest way to do it:
我认为这是最好和最简单的方法：
if (!(isset($action) && ($action == "add" || $action == "delete")))
回答by Your Common Sense
Not an answer, but just for the sake of code formatting
不是答案，只是为了代码格式
if((isset($_GET['a'])) $action=$_GET['a']; else $action ="";
if(!($action === "add" OR $action === "delete")){
header("location: /index.php");
exit;
}
Note the exit;statement after header(). That's the important thing. header() does not terminate script execution.
请注意exit;header() 之后的语句。这才是最重要的。header() 不会终止脚本执行。
