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

How is this not a bug? The original array contains string values and the result is not correct.  This is not a complicated situation. Is there a flag that is need to make this simple case work as the documentation implies that it should?  If so, then the documentation should be improved.

> Note that keys are preserved. If multiple elements compare equal under the given
> sort_flags, then the key and value of the first equal element will be retained.

In this case there are no keys in the array. It is an array of elements that are just simple strings where each has a numeric index provided by PHP when the element is assigned to the array.  The resulting array also contains many elements that have unset values and produce errors of bad index when attempting access thru the array sequentially with successive numeric indexes.  If the count of elements in the array shows 30 elements, then each element index 0 thru 29 should have a value.  Otherwise, the new array makes no sense.

> In this case there are no keys in the array

there is nothing like an array without keys in PHP

> each has a numeric index provided by PHP when the element is assigned

so the array has keys as *every* array has by defintion because hash tables are impossible without

php > print_r(['a', 'b', 'c']);
Array
(
    [0] => a
    [1] => b
    [2] => c
)

> If the count of elements in the array shows 30 elements, 
> then each element index 0 thru 29 should have a value.  

and here we are back at "Note that keys are preserved"

> Otherwise, the new array makes no sense

again: "Note that keys are preserved"

normal, expected behavior

php > print_r(['x', 'a', 'x', 'b', 'a', 'b']);
Array
(
    [0] => x
    [1] => a
    [2] => x
    [3] => b
    [4] => a
    [5] => b
)

php > print_r(array_unique(['x', 'a', 'x', 'b', 'a', 'b']));
Array
(
    [0] => x
    [1] => a
    [3] => b
)

and to finish that discussion: "then each element index 0 thru 29 should have a value.  Otherwise, the new array makes no sense" is nonsense because in most caes nobody iterates arrays with a for-each loop or care about the numeric keys at all

if you care in a specific usecaes just call array_values(9 and you are done, but don#t insist in the performance penalty doing that behind the scenes all the time even if it don't matter for the code

the language has everything you need, just use it

php > print_r(array_values(array_unique(['x', 'a', 'x', 'b', 'a', 'b'])));
Array
(
    [0] => x
    [1] => a
    [2] => b
)