学习PHP，布尔代数

问题描述：

So i've just started learning PHP and i came across a part i didn't quite understand.

The book gave me three lines.

&& and true && true=true, every other combination results in false.

|| or false || false=false, every other combination results in true.

XOR or false XOR true=true, every other combination results in false.

If anyone can clarify what this means i would very much appreciate it.

Edit

the following is text above my previous part.

Every equation yields a value: either true(1) or false(0).

echo true + true + false

This results in a value of 2 (1 + 1 + 0).

所以我刚刚开始学习PHP，我遇到了一个我不太了解的部分。 p >

这本书给了我三行。 p>

＆amp;＆amp; 和真实＆amp;＆amp; true = true，其他每个组合都会产生 false em>。 p>
|| 或假|| false = false，其他每个组合都会产生 true em>。 p>
XOR或false XOR true = true，其他每个组合都会产生 false em> 。 p> blockquote>
如果有人能澄清这意味着什么我会非常感激。 p>

编辑 strong > p>
以下是我上一部分的文字。 p>

每个等式产生一个值：true（1）或false（0）。 p>

echo true + true + false code> pre> 这导致值为2（1 + 1 + 0 ）。 p> div>

答

There are three boolean operators mentioned there: && (logical AND), || (logical OR), and XOR (well, it's logical XOR, or 'exclusive OR'). All of these are binary ones - they take two operands. Its result, apparently, is a boolean value - either true or false.

Now, they function as follows:

&& will only result in true if both its operands evaluate to true, otherwise the result will be false
|| will only result in false if both its operands evaluate to false, otherwise the result will be true
XOR will result in false if its operands evaluate to the same value - be it true or false, doesn't matter. But if one operand evaluates to false, and another to true, the result is true.

Now, on the second part of your question: this...

echo true + true + false;

... doesn't have anything to do with boolean algebra. All the operands of + are cast to the numeric type first, by the rules described in Type Juggling section of the PHP documentation. In short, true is converted to 1, false to 0; the result - 1 + 1 + 0, or 2, is printed out.

答

It is referring to the fact that and and && etc. have different operator precedence.

Namely, and or or function differently than && and || with assignment statements:

$f = false or true;

Also, the operators are short-circuit operators, so if you have something that evaluates to false as the first operand with either and or && then the entire expression will immediately evaluate to false without evaluating any other operands.

http://www.php.net/manual/en/language.operators.logical.php

学习PHP，布尔代数

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