Bug or hack? $GLOBALS

Here is a possible explanation:

We know that foreach always loops over a copy of the array if it is not referenced:

Unless the array is referenced, foreach operates on a copy of the specified array and not the array itself. foreach has some side effects on the array pointer.

That means that the internal pointer of the original array is not changed and key() will always return the same value (as we can see when we comment out the line). And indeed if we do a var_dump($GLOBALS), we get:

 ["items"]=>  array(7) {    [0]=>    string(3) "one"    [1]=>    string(3) "two"    [2]=>    string(5) "three"    [3]=>    string(4) "four"    [4]=>    string(4) "five"    [5]=>    string(3) "six"    [6]=>    string(5) "seven"  }

(no reference)

But as soon as we generate a reference to the array (with $alter), $GLOBALS['items'] becomes a reference too, because both entries have to point to the same array:

 ["items"]=>  &array(7) {    [0]=>    string(3) "one"    [1]=>    string(3) "two"    [2]=>    string(5) "three"    [3]=>    string(4) "four"    [4]=>    string(4) "five"    [5]=>    string(3) "six"    [6]=>    string(5) "seven"  }  ["alter"]=>  &array(7) {    [0]=>    string(3) "one"    [1]=>    string(3) "two"    [2]=>    string(5) "three"    [3]=>    string(4) "four"    [4]=>    string(4) "five"    [5]=>    string(3) "six"    [6]=>    string(5) "seven"  }

Hence, the foreach loop does iterate over the original array and changes the internal pointer, which affects key().

To sum up: It is a problem with references, not with $GLOBALS.