Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

This is to be expected. Indexing of any array always starts 
from element 0 and is not carried over across array 
dimensions. 

Maybe the example I used was not understood well.
I don't want indexing to be propagated thru array dimensions. IMHO array_merge_recursive is indexing in two different ways depending of the level of recursion. This is better example:

Reproduce code:
---------------

    $ar1 = array(10=>10, 7=>7);
    $ar2 = array(10=>10, 7=>7);

    print_r(array_merge_recursive($ar1, $ar2)); 

    $ar1 = array("a" => array(10=>10, 7=>7));
    $ar2 = array("a" => array(10=>10, 7=>7));

    print_r(array_merge_recursive($ar1, $ar2)); 

    $ar1 = array("a" => array("a" => array(10=>10, 7=>7)));
    $ar2 = array("a" => array("a" => array(10=>10, 7=>7)));

    print_r(array_merge_recursive($ar1, $ar2)); 


Actual result:
--------------

Array
(
    [0] => 10
    [1] => 7
    [2] => 10
    [3] => 7
)
Array
(
    [a] => Array
        (
            [10] => 10
            [7] => 7
            [11] => 10
            [12] => 7
        )

)
Array
(
    [a] => Array
        (
            [a] => Array
                (
                    [10] => 10
                    [7] => 7
                    [11] => 10
                    [12] => 7
                )

        )

)

Expected result is the same as above.

From the manual:

"If the input arrays have the same string keys, then the values for these keys are merged together into an array, and this is done recursively, so that if one of the values is an array itself, the function will merge it with a corresponding entry in another array too. If, however, the arrays have the same numeric key, the later value will not overwrite the original value, but will be appended."

"If the input arrays have the same string keys, then the values for these keys are merged together into an array ..."
Of course that the values are merged. But the way in which are merged is IMHO wrong. Keys are reindexed starting from 0 in the first level of recursion but in the next levels reindexing is different. As one can see on the output of an example above. Why?

There is no bug in this.

I have also come across this and believe it to be a bug. As per the documentation, array_merge is supposed to re-index numeric keys from 0. One would expect array_merge_recursive to re-index numeric keys from 0 at every level of recursion, however, it only re-indexes numeric keys at the first level. A recursive function that behaves differently at different levels of recursion is objectively a bug.

Maybe a simpler example will illustrate it better:

Reproduce code:
---------------
$ar1 = array(1 => array(2 => 3));
print_r(array_merge_recursive($ar1));

Expected result:
----------------
Array
(
    [0] => Array
        (
            [0] => 3
        )

)

Actual result:
----------------
Array
(
    [0] => Array
        (
            [2] => 3
        )

)