php为什么或运算符返回0为真

Question

In the following code:

$a = 0 or 1;
$b = 0 || 1; 
echo "$a, $b"; // 0, 1

Why does $a equal zero, I thought or and || were interchangeable in PHP? What exactly is going on with the or statement to make it return 0 ?

I would have assume both results would have been 1 making it echo 1, 1 .

Answer 1

or is lower precedence than = which is lower precedence than ""

So your code is equivalent to:

($a = 0) or 1;
$b = (0 || 1); 

See the precedence table in the PHP manual.

Answer 2

It's because of the rules of precedence in PHP. The assignment = operator has a lower precedence than the logical || operator, but a higher precedence than the logical OR operator. See here: http://php.net/manual/en/language.operators.precedence.php

Answer 3

Its because of the order of precedence

// The result of the expression (false || true) is assigned to $e
// Acts like: ($e = (false || true))
$e = false || true;

// The constant false is assigned to $f and then true is ignored
// Acts like: (($f = false) or true)
$f = false or true;

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