PHP add missing keys between arrays

I have the following code:

$a = array('a' => 'some value', 'b' => 'some value', 'c' => 'some value');
$b = array('a' => 'another value', 'd' => 'another value', 'e' => 'another value', 'f' => 'another value');
$c = array('b' => 'some more value', 'x' => 'some more value', 'y' => 'some more value', 'z' => 'some more value');

$d = array($a, $b, $c);
Printing $d will output:

Array
(
    [0] => Array
        (
            [a] => some value
            [b] => some value
            [c] => some value
        )

    [1] => Array
        (
            [a] => another value
            [d] => another value
            [e] => another value
            [f] => another value
        )

    [2] => Array
        (
            [b] => some more value
            [x] => some more value
            [y] => some more value
            [z] => some more value
        )

)
How would I go about combining the array keys and end up with the following output?

Array
(
    [0] => Array
        (
            [a] => some value
            [b] => some value
            [c] => some value
            [d] =>
            [e] =>
            [f] =>
            [x] =>
            [y] =>
            [z] =>
        )

    [1] => Array
        (
            [a] => another value
            [b] =>
            [c] =>
            [d] => another value
            [e] => another value
            [f] => another value
            [x] =>
            [y] =>
            [z] =>
        )

    [2] => Array
        (
            [a] =>
            [b] => some more value
            [c] =>
            [d] =>
            [e] =>
            [f] =>
            [x] => some more value
            [y] => some more value
            [z] => some more value
        )

)

Answer1:
Yes you can use array_merge in this case:

$a = array('a' => 'some value', 'b' => 'some value', 'c' => 'some value');
$b = array('a' => 'another value', 'd' => 'another value', 'e' => 'another value', 'f' => 'another value');
$c = array('b' => 'some more value', 'x' => 'some more value', 'y' => 'some more value', 'z' => 'some more value');
$d = array($a, $b, $c);
$keys = array();
foreach(new RecursiveIteratorIterator(new RecursiveArrayIterator($d)) as $key => $val) $keys[$key] = '';
$data = array();
foreach($d as $values) {
    $data[] = array_merge($keys, $values);
}

echo '<pre>';
print_r($data);

Answer2:
$a = array('a' => 'some value', 'b' => 'some value', 'c' => 'some value');
$b = array('a' => 'another value', 'd' => 'another value', 'e' => 'another value', 'f' => 'another value');
$c = array('b' => 'some more value', 'x' => 'some more value', 'y' => 'some more value', 'z' => 'some more value');

$d = array_merge(array_merge($a, $b),$c);

foreach($d as $k=>$v){
    $aN[$k] = isset($a[$k])?$a[$k]:'';
    $bN[$k] = isset($b[$k])?$b[$k]:'';
    $cN[$k] = isset($c[$k])?$c[$k]:'';
}
$dN = array($aN, $bN, $cN );