The problem of using & for key values ​​in foreach loop in php

The code is as follows:

<code>$a = [1, 2, 3];
foreach($a as $key => &$value) {
    $value = 5;
}
var_dump($a);</code>

The final output result is as follows:

<code>array(3) { [0]=> int(5) [1]=> int(5) [2]=> &int(5) }</code>

Question: Why does the address character appear before the last key value?

Reply content:

The code is as follows:

<code>$a = [1, 2, 3];
foreach($a as $key => &$value) {
    $value = 5;
}
var_dump($a);</code>

The final output result is as follows:

<code>array(3) { [0]=> int(5) [1]=> int(5) [2]=> &int(5) }</code>

Question: Why does the address character appear before the last key value?

The & appearing in this var_dump means that you can use another variable to change its value.
Like you

<code class="php">$b = &$a[0];
var_dump($a);</code>

When printing here, an & symbol will appear in the first key value, indicating that you can use other variables ($b) to change its value.

Let’s first understand the basic usage of &.

<code class="php">$b = $c = 1;
$a = &$b;
$a = 2;
echo "b=$b,c=$c\n";
$a = &$c;
$a = 100;
$a = 200;
echo "b=$b,c=$c\n";
unset($a);
$a = 500;
echo "b=$b,c=$c\n";</code>

After $a is specified as a reference to $b above, unless it is specified again as another reference or unset($a), changing $a will change the value of the corresponding $b.

The same is true for your foreach. If you break the loop into single steps, it will look like this:

<code class="php">$value = &$a[0];
$value = 5;
$value = &$a[1];
$value = 5;
$value = &$a[2];
$value = 5;</code>

When the loop runs to the end, $value is a reference to $a[2], so it is equivalent to this form:

<code class="php">$a[0] = 5;
$a[1] = 5;
$value = &$a[2];
$a[2] = 5;
var_dump($a);</code>

It is not difficult to understand why var_dump prints an & symbol on the last key-value pair.

---

Under normal circumstances, this will not cause any big problems, but due to the scope problem of foreach, $value can still be used after the loop exits, so some incredible bugs will appear.

Like this

<code class="php">$a = [1, 2, 3];
foreach($a as $key => &$value) {
    $value = $value+5;
}
var_dump($a); // [6,7,8]
foreach($a as $value){}
var_dump($a); // [6,7,7]</code>

After adding an empty foreach, $a becomes unreasonable. This bug is incredible. The reason is also because the $value in the empty foreach is a reference to the top &a[2].

The empty foreach is equivalent to this:

<code class="php">$value = &$a[2];
$value = $a[0];
$value = $a[1];
$value = $a[2];</code>

Did you notice that since $value is a reference to $a[2], the above can be rewritten like this:

<code class="php">$a[2] = $a[0];
$a[2] = $a[1];
$a[2] = $a[2];</code>

$a[2] is constantly changing in the empty foreach, and since $a[2] has become the value of $a[1], $a[2] = $a[2]; has no effect. , the value is still $a[1];

This incredible bug is caused by the scope of the variable $value, so either change the name or unset $value first.

<code class="php">$a = [1, 2, 3];
foreach($a as $key => &$value) {
    $value = $value+5;
}
var_dump($a);
// 要么unset
unset($value);
// 要么foreach里不要用上面同名的$value,改为$value2222
foreach($a as $value2222){}
var_dump($a);</code>

First of all, I want to say that this is a very, verygood question. I believe the questioner is not unable to distinguish between quotes and non-quotes.
I did two tests first:

<code>$a = [1, 2, 3];
foreach($a as $key => &$value) {
    $value = 5;
}
foreach($a as $key => $value) {
    $value = 6;
}
var_dump($a);</code>

What do you guess the output is?

<code>array(3) {
  [0]=>  int(5)
  [1]=>  int(5)
  [2]=>  &int(6)
}</code>

Another test is:

<code>$a = [1, 2, 3];
foreach($a as $key => &$value) {
    $value = 5;
}
foreach($a as $key => $value2) {
    $value2 = 6;
}
var_dump($a);</code>

The output this time looks much more normal:

<code>array(3) {
  [0]=>  int(5)
  [1]=>  int(5)
  [2]=>  &int(5)
}</code>

So, this problem should be a pitfall of PHP global variables.