Description:
------------
The behaviour of array_map for string keys is not documented.

If the array argument  contains string keys then the returned array will contain string keys if exactly one array is passed. If more than one argument is passed then the returned array always has integer keys.

The following code demonstrates: 

$ cat 1.php
<?php
$arr = array("stringkey" => "value");
function cb1 ($a) {return array ($a);};
function cb2 ($a,$b) {return array ($a,$b);};
var_dump( array_map("cb1", $arr));
var_dump( array_map("cb2", $arr,$arr));
var_dump( array_map(null,  $arr));
var_dump( array_map(null, $arr, $arr));
?>
 
array(1) {
  ["stringkey"]=>
  array(1) {
    [0]=>
    string(5) "value"
  }
}
array(1) {
  [0]=>
  array(2) {
    [0]=>
    string(5) "value"
    [1]=>
    string(5) "value"
  }
}
array(1) {
  ["stringkey"]=>
  string(5) "value"
}
array(1) {
  [0]=>
  array(2) {
    [0]=>
    string(5) "value"
    [1]=>
    string(5) "value"
  }
}