MonkeyZeus MonkeyZeus - 2 months ago 5
PHP Question

Unexpected behavior when assigning multiple variables in if() block

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

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()) {
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