How does PHP foreach actually work

0 votes

Let me start by stating that I am familiar with foreach's functions and usage. I don't want any replies that say, "This is how you loop an array with foreach," because this question is about how it actually operates.

I believed for a very long time that foreach just applied to the array itself. After discovering numerous references to the fact that it functions with a copy of the array, I came to the conclusion that this was the end of the matter. However, I recently engaged in a conversation on the subject and discovered through some experiments that this was not quite accurate.

Test case 1:

foreach ($array as $item) {
  echo "$item\n";
  $array[] = $item;
}
print_r($array);

/* Output in loop:    1 2 3 4 5
   $array after loop: 1 2 3 4 5 1 2 3 4 5 */

Since we are continuously adding items to the array while the loop is running, this clearly demonstrates that we are not interacting directly with the source array. But to be certain that this is the case:

Test case 2:

foreach ($array as $key => $item) {
  $array[$key + 1] = $item + 2;
  echo "$item\n";
}

print_r($array);

/* Output in loop:    1 2 3 4 5
   $array after loop: 1 3 4 5 6 7 */

This backs up our initial conclusion, we are working with a copy of the source array during the loop, otherwise we would see the modified values during the loop. But...

If we look in the manual, we find this statement:

When foreach first starts executing, the internal array pointer is automatically reset to the first element of the array.

Test case 3:

// Move the array pointer on one to make sure it doesn't affect the loop
var_dump(each($array));

foreach ($array as $item) {
  echo "$item\n";
}

var_dump(each($array));

/* Output
  array(4) {
    [1]=>
    int(1)
    ["value"]=>
    int(1)
    [0]=>
    int(0)
    ["key"]=>
    int(0)
  }
  1
  2
  3
  4
  5
  bool(false)
*/

The fact that the source array pointer is at the end of the array at the end of the loop indicates that, while not working directly with the source array, we are working directly with it. However, if this were the case, test case 1 would loop indefinitely.

Test case 4:

foreach ($array as $key => $item) {
  echo "$item\n";
  each($array);
}

/* Output: 1 2 3 4 5 */

Test case 5:

foreach ($array as $key => $item) {
  echo "$item\n";
  reset($array);
}

/* Output: 1 2 3 4 5 */

Question

What is going on here? My C-fu is not good enough for me to able to extract a proper conclusion simply by looking at the PHP source code, I would appreciate it if someone could translate it into English for me.

It seems to me that foreach works with a copy of the array, but sets the array pointer of the source array to the end of the array after the loop.

  • Is this correct and the whole story?
  • If not, what is it really doing?
  • Is there any situation where using functions that adjust the array pointer (each(), reset() et al.) during a foreach could affect the outcome of the loop?

 

Aug 7, 2022 in PHP by Kithuzzz
• 38,010 points
625 views

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