Analysis of problems arising from foreach reference in php

1, foreach is the loop output of php on the array.

Example:

$arr = array("1"=>"111","2"=>"222","3"=>"333");
foreach($arr as $key=>$value)
{
  echo $key."=>".$value."\n";
}

The result is as follows:

1=>111
2=>222
3=>333

2, slightly modified:

foreach($arr as $key=>$value)
{
//echo $key."=>".$value."\n";
$key = &$arr[$key];
}
print_r($arr);

The result is as follows:

Array
(
    [1] => 2
    [2] => 3
    [3] => 333
)

Code explanation :

We found that the original array was modified. Why? Let's study it.

The key part in the code is: $key = &$arr[$key];

$key is the reference of $arr[$key], that is , when $key is modified, $arr[$key] is also modified to the corresponding value.

First of all, we need to understand the principle of foreach. It assigns the values ​​of the array to $key and $value respectively;

So, $key and $value are also ordinary ones. variable.

Next analysis, in the first loop, $key = &$arr[$key]; means &$arr[1] points to the variable $key.

When the foreach loop reaches the second pass, first, $key is assigned a value of 2. At this time, pay attention again, $key = &$arr[$key];

The result is :$arr[1] is assigned the new $key at this time, which is 2.

After the second loop ends, the original array becomes:

Array
(
    [1] => 2
    [2] => 222
    [3] => 333
)

Similarly, After the third cycle ends, it will be:

Array
(
    [1] => 2
    [2] => 3
    [3] => 333
)

At this point, it has been roughly explained.

3, in order to understand more clearly, the assignment process in foreach, we can do this:

$arr = array("1"=>"111","2"=>"222","3"=>"333");
foreach($arr as $key=>$value)
{
$key  = &$arr[$key];
$key = "hello";
unset($key);
print_r($arr);
}

The result is as follows:

Array
(
    [1] => hello
    [2] => 222
    [3] => 333
)
Array
(
    [1] => hello
    [2] => hello
    [3] => 333
)
Array
(
    [1] => hello
    [2] => hello
    [3] => hello
)

Code explanation:

We directly assign the value "hello" to $key in each loop, so as not to affect it, and then release the $key variable.

This should make it clearer.

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