Analysis of problems in the foreach reference in PHP

1,foreach is the cyclic output of an array of PHP.

Example:

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

The results are as follows:

1=>1112=>2223=>333

2, make a slight change:

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

The results are as follows:

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

Code Explanation:

We found that the original array was modified, for what? Let's take a look at it.

The key points in the code are: $key = & $arr [$key];

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

First, let's look at the principle of foreach, which is to assign values to $key and $value, respectively;

So, $key, $value is also a common variable.

Then analysis, the first cycle, $key = & $arr [$key], meaning & $arr [1] point to the variable $key.

When the Foreach loop is repeated to the second pass, first, $key is assigned a value of 2, at which point, $key = & $arr [$key];

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

At the end of the second loop, the original array becomes:

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

Similarly, at the end of the third cycle, the following is:

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

By this, it is probably clear that.

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);p rint_r ($arr);}

The results are as follows:

Array (    [1] = Hello    [2] = 222    [3] = = 333) Array (    [1] = = Hello    [2] = = Hello [    3] =&G T 333) Array (    [1] = = Hello    [2] = = Hello    [3] = = Hello)

Code Explanation:

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

This should be, it is more clear.