Php foreach- Why it doesn't override the array value while iterating?

Question

I have a doubt in php foreach iteration.

Please look my below code.

CODE :

   $arr=array(1,2,3);

    echo '$arr value : '. $arr;

    echo '<br>';

    foreach($arr as $arr){
        echo $arr.'<br>';
    }
    echo '$arr value : '. $arr;

OUTPUT :

$arr value : Array
1
2
3
$arr value : 3

While iterating array i used same array name to value key foreach($arr as $arr) but it works fine. How it is working?.

Why it doesn't override the array value while iterating?

I want to know how foreach iteration is working.

Please Help me!

Thanks in advance Logan

Answer 1

Why it doesn't override the array value while iterating?

foreach will work on a copy of the original array so things you change in the foreach loop will not change the values you are looping over.

Answer 2

PHP uses a copy on write or lazy copy mechanism with reference counting for memory handling.

In the first stage of foreach iteration, $arr is "soft copied" (no actual copy, but only the refcount of the zval of $array is increased). checkout variable container and label of PHP variables.

In our case,

• it first keep a reference to actual $arr variable, and the first value of array is fetched from the reference point.
• For later use first array element is assigned to second $arr foreach($arr as $arr)
• In the next iteration, second array value is fetched from previously saved reference point, and that value is assigned to same $arr and continues.
• As value of $arr is changed inside loop, you cannot print that array outside loop using $arr variable.

Hope you can understand...

For an in-depth study checkout http://www.php.net/manual/en/internals2.opcodes.fe-fetch.php and http://www.php.net/manual/en/internals2.opcodes.fe-reset.php

Interestingly you can play with this too...

current() inside a foreach loop will always returns the same element!!!

Code

$source = array(10, 20, 30);
foreach ($source as $value) {
    echo 'Value is ', $value, ', and current() is ', current($source), '<br>';
}

Output

Value is 10, and current() is 20
Value is 20, and current() is 20
Value is 30, and current() is 20

Answer 3

foreach ($arr as $arr) {
    // code
}

This may be confusing to you because your mental model of how this works is:

1. PHP takes the current element out of $arr and advances the array pointer
2. it assigns the value to $arr
3. it executes // code
4. it repeats from step one

What it actually does though is this (or at least an approximate simplification of it):

1. PHP starts the foreach operation, taking the array value that is associated with $arr
2. it puts this array somewhere internally, let's say a var iterator
3. it takes the current element from var iterator and advances the iterator
4. it assigns the value it took to $arr in your script
5. it executes // code
6. it repeats from step 3

In other words, it does not take the next value from $arr on every iteration, it only used this once to get the initial iterator. The actual thing that it iterates over is stored internally and is not associated with $arr anymore. That means $arr is free for other purposes in your script and PHP is happy to reuse it on each iteration. After the loop is done, the last element remains in $arr.

If you know anonymous functions, maybe this illustrates better why this works the way it works:

foreach($arr, function ($arr) {
    // code
});

The implementation of such a foreach function can be simply this:

function foreach(array $array, callable $code) {
    while (list(, $value) = each($array))  {
        $code($value);
    }
}