Problems with & of key values in the Foreach Loop in PHP

The code is as follows:

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

The final output results are as follows:

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

Question: Why do I get an address character before the last key value?

Reply content:

The code is as follows:

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

The final output results are as follows:

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

Question: Why do I get an address character before the last key value?

The & represented in this var_dump you can change its value with a different variable.
Like you do.

$b = &$a[0];var_dump($a);

When printed here, the & symbol appears in the first key value, indicating that you can change its value with other variables ($b).

Let & 's take a look at the basic usage.

$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";

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

For you this foreach, is the same truth, you take the loop apart, this is the way:

$value = &$a[0];$value = 5;$value = &$a[1];$value = 5;$value = &$a[2];$value = 5;

The loop runs to the last time, $value is a reference to $a[2], so it is equivalent to this form:

$a[0] = 5;$a[1] = 5;$value = &$a[2];$a[2] = 5;var_dump($a);

It is not difficult to understand why Var_dump will print a & symbol on the last key-value pair.

In general, this will not cause any major problems, but because of the scope of the foreach problem, will cause the loop exit after the $value can still be used, so there will be some strange bugs.

Like this.

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

After adding an empty foreach, it turns out that $ A is unreasonable, and the bug is not surprising, because the $value in the empty foreach is the reference to the top &a[2].

The empty foreach is equivalent to this:

$value = &$a[2];$value = $a[0];$value = $a[1];$value = $a[2];

Notice that because $value is a reference to $a[2], the above rewrite is this:

$a[2] = $a[0];$a[2] = $a[1];$a[2] = $a[2];

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

This strange bug is due to the scope of the variable $value, so either change the name, or first give the $value to unset off.

$a = [1, 2, 3];foreach($a as $key => &$value) {    $value = $value+5;}var_dump($a);// 要么unsetunset($value);// 要么foreach里不要用上面同名的$value,改为$value2222foreach($a as $value2222){}var_dump($a);

First of all, I would like to say that this is a very, very good question. I believe that the main problem is not the separation of reference and non-reference.
I did a little bit of two Tests first:

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

What do you guess the output is?

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

Another test is:

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

This time the output looks a lot more normal:

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

So, the problem should be a hole in the PHP global variable.

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