PHP Circular Reference

Yesterday, I encountered a quoted question.

The result is, the first time I also take it for granted.

But the actual results made me surprised. The correct result for [2,3,3]

After some thought and discussion I probably analyzed the flow of its operation and why it was [2,3,3] the result
First, the reference is two or more variables pointing to the same memory address as

$a = 5;
$b = & $a;
So they point to the same memory address whether you change a $ A or a $b value, the other value changes.
Now let me go back to that loop

$arr = [n/a];
foreach ($arr as $key = = $value) {
$value = & $arr [$key];
Print_r ($arr);
}

As you can see from the code, $value points to the address of each element in the array, and if the $value value changes, the value in the array will change.

First cycle
$keyt =0 $value = 1 1 = $arr [0] $value This variable already points to the first element in the $arr array where the position is 1 of the memory location
So when a loop finishes printing the array is

Second cycle
This is the point, because the first loop $value this variable already points to the first element in the $arr array where the position is 1 of the memory location, when the second loop is just beginning, not yet running to $value = & $arr [$key]; this line, at which time the second loop has already put $ Value is changed to 2 because the first loop $value to the position of the array $arr element 1, so when the second loop starts $value the value becomes 2, so the first element in $arr becomes 2 and continues down to $value = & $arr [$key]; 2 = & $arr [1] At this point the address $value to $arr the second element to the memory location, while the second element is assigned a value of 2

The third loop $value already points to the $arr of the second element of the array, so the loop has just begun to $value value is assigned 3, that is, the second element of the array is 3, and then run down $value = & $arr [$key]; 3= & $arr [2] The third element of the vegetarian group is 3, $value the position of the third element that points to the array

The end result of this entire array loop is [2,3,3] the same as if the array adds another 4 to the inevitable [2,3,4,4]

Summarize
At the end of the first loop, a multi-pair memory address mapping is established, and each loop will change the value of the $value corresponding memory address first, that is, the value of the array element, and then $value the memory address of the next element of the array.

PHP Circular Reference