为什么 if($x = 1 && $x == 1) 抛出错误但 if ($x == 1 && $x = 2) 不在 php 中？

Question

In php the following code gives a warning of undefined variable $x :

if($x = 1 && $x == 1)

I thought it was equivalent to if( ($x = 1) && ($x == 1) ) , but that's not the case. I've been told, it's because && has higher precedence than = , which causes the expression to get converted to:

if($x = (1 && ($x == 1)))

So far so good, but now consider:

$x=1; if($x == 1 && $x = 2)

This doesn't throw error. Why doesn't it get converted to:
$x=1; if( (($x == 1) && $x) = 2 )

I've been told thats due to = being right assosiative, buthttps://www.php.net/manual/en/language.operators.precedence.php says When operators have equal precedence their associativity decides how the operators are grouped. . Here we have = , && and == all being of different precedence.

PS; My actual code is if($result != false && $res = $stmt->get_result()) , which has been copied from some other reputable source, so seems like not using unneeded parenthesis is common in php.

Answer 1

I've played with several conditions and below is what I've got.

First, let's consider that we init $x before if statements to avoid undefined variable notice.

Second, let's confirm theprecedence for operators:

1. == is applied 1st
2. && is applied 2nd
3. = is applied 3rd

---

This returns true :

$x = 1;
if ($x = 1 && $x == 1) {
    echo 'true';
} else {
    echo 'false';
}

It goes like ($x = (1 && ($x == 1))) -> ($x = (1 && true)) -> ($x = true) -> true .

---

If we compare $x to another value than the assigned one we will get false :

$x = 1;
if ($x = 2 && $x == 2) {
    echo 'true';
} else {
    echo 'false';
}

It goes like ($x = (2 && ($x == 2))) -> ($x = (2 && false)) -> ($x = false) -> false .

---

The last one returns true :

$x = 1;
if ($x == 1 && $x = 2) {
    echo 'true';
} else {
    echo 'false';
}

It goes like ((($x == 1) && $x) = 2) -> ((true && $x) = 2) -> (true = 2) -> true .

The last comparison can't be interpreted by PHP so it's an approximate view.

It looks like the last action (true = 2) totally depends on the left operand . If we put $x = 2; we will get (false = 2) -> false .

---

I'm not sure about the last one and here is the only place were some mistakes can happen.

Otherwise, it looks like precedence works as expected.

Anyway, I always put parenthesis for an assignment action inside if operator (especially inside ternary if) to be sure that I will get what I expect. I don't think this affects performance or readability too much, but it may prevent some logical errors.

---

UPDATE :

Concering your code if($result != false && $res = $stmt->get_result()) it's not correct to compare it to if($x == 1 && $x = 2) because in your code are two different variables.

In this case logical operator will not call the second part at all if the fisrt one is false , see the 1st example here

UPDATE-2 :

After the discussion under this answer we can see that the last conditions ($x == 1 && $x = 2) work like like (($x == 1) && ($x = 2)) -> ((1 == 1) && 2) -> true and $x becomes 2 after it.