Lets consider that we have following arrays, $ar1 and $ar2:

// first array
$ar1 = array();
$ar1[1] = 'val1';
$ar1[2] = 'val2';

// second array
$ar2 = array('key' => '');

Than we have resulting array $res. Let's lookwhat happend after array_merge() call:

$res = array_merge($ar1, $ar2);
var_dump($res);

Resulting array is described here:

array(3) {
  [0]=>
  string(4) "val1"
  [1]=>
  string(4) "val2"
  ["key"]=>
  string(0) ""
}

Numeric keys 1 and 2 was recoded to 0 and 1. Looks like bug, isn't it?

I refer to the documentation, but there is nothing written about the numeric keys redefining. An exaple there (in documentation) shows example, where only numeric keys started from 0 are used, so no such change happened.