在if（）块中分配多个变量时出现意外行为

Question

In the spirit of seeing Short-circuit evaluation like Python's "and" while storing results of checks I decided to see how this could be best solved in PHP but I've run into an issue.

unexpected

<?php
function check_a()
{
    return 'A';
}
function check_b()
{
    return 'B';
}
function check_c()
{
    return 'C';
}

if($a = check_a() && $b = check_b() && $c = check_c())
{
    var_dump($a);
    var_dump($b);
    var_dump($c);
}

Results in:

bool(true)
bool(true)
string(1) "C"

---

code for what I wanted to happen

<?php
function check_a()
{
    return 'A';
}
function check_b()
{
    return 'B';
}
function check_c()
{
    return 'C';
}

// if(($a = check_a()) && ($b = check_b()) && $c = check_c()) // equivalent to line below
if(($a = check_a()) && ($b = check_b()) && ($c = check_c()))
{
    var_dump($a);
    var_dump($b);
    var_dump($c);
}

Results in:

string(1) "A"
string(1) "B"
string(1) "C"

---

Why does the unexpected example act in this way?

Answer 1

This is a question of operator precedence . An assignment expression returns the assigned value , so you would expect to get A and B for the first two operations. The reason you're getting boolean true instead is that the && operator has a higher precedence than the assignment operator, so in the original expression

$a = check_a() && $b = check_b() && $c = check_c()

$a gets the value of check_a() && $b = check_b() && $c = check_c() ,

$b gets the value of check_b() && $c = check_c() ,

and $c gets the value of check_c() .

The expressions check_a() && $b = check_b() && $c = check_c() , and check_b() && $c = check_c() return boolean true , because the use of the && operator causes the expressions to be evaluated as booleans, and all components of the expressions joined by && evaluate to true .

To get the results you expect, you can add parentheses as you did, or you can use the and logical operator instead of && , because it has a lower precedence than the assignment operator.

if($a = check_a() and $b = check_b() and $c = check_c()) {

Answer 2

The result of a boolean && is a bool! && binds stronger than = . Therefore $b gets assigned ('B' && $c) which is a bool... Same obviously with $a ...