How does PHP 'foreach' actually work?

Question

Let me prefix this by saying that I know what foreach is, does and how to use it. This question concerns how it works under the bonnet, and I don\'t want any an

Answer 1

NOTE FOR PHP 7

To update on this answer as it has gained some popularity: This answer no longer applies as of PHP 7. As explained in the "Backward incompatible changes", in PHP 7 foreach works on copy of the array, so any changes on the array itself are not reflected on foreach loop. More details at the link.

Explanation (quote from php.net):

The first form loops over the array given by array_expression. On each iteration, the value of the current element is assigned to $value and the internal array pointer is advanced by one (so on the next iteration, you'll be looking at the next element).

So, in your first example you only have one element in the array, and when the pointer is moved the next element does not exist, so after you add new element foreach ends because it already "decided" that it it as the last element.

In your second example, you start with two elements, and foreach loop is not at the last element so it evaluates the array on the next iteration and thus realises that there is new element in the array.

I believe that this is all consequence of On each iteration part of the explanation in the documentation, which probably means that foreach does all logic before it calls the code in {}.

Test case

If you run this:

 1,
        'bar' => 2
    );
    foreach($array as $k=>&$v) {
        $array['baz']=3;
        echo $v." ";
    }
    print_r($array);
?>

You will get this output:

1 2 3 Array
(
    [foo] => 1
    [bar] => 2
    [baz] => 3
)

Which means that it accepted the modification and went through it because it was modified "in time". But if you do this:

 1,
        'bar' => 2
    );
    foreach($array as $k=>&$v) {
        if ($k=='bar') {
            $array['baz']=3;
        }
        echo $v." ";
    }
    print_r($array);
?>

You will get:

1 2 Array
(
    [foo] => 1
    [bar] => 2
    [baz] => 3
)

Which means that array was modified, but since we modified it when the foreach already was at the last element of the array, it "decided" not to loop anymore, and even though we added new element, we added it "too late" and it was not looped through.

Detailed explanation can be read at How does PHP 'foreach' actually work? which explains the internals behind this behaviour.