Insert just the array key without value

I have the following code in PHP:

$keys = array('a', 'b', 'c');
$data = array();
$data[0] = array_fil开发者_如何学Pythonl_keys($keys,'');

After some follow-up code, when the array set is empty, it returns:

a,b,c
,,,

I've slimmed down to the problem is $data[0] = array_fill_keys($keys,'');

What can I do to have the result as:

a,b,c

Thanks.

    $keys = array('a', 'b', 'c');
    $data = array();
    echo $data[0] = $keys[0];
    echo $data[1] = $keys[1];
    echo $data[2] = $keys[2];

or 

    $keys = array('a', 'b', 'c');
    $data = array();
    foreach ($keys as $value) {
    echo $value . ',';
    }

it will output the letter a and a,b,c, rtrim() function remove chars from right side of the strig, for left side use ltrim()