The writing ( with foreach) of the keys of an array of objects to a new array produces fatal error

I have the following problem, i've queried the attached media to a post with:

function abc_attachments( $post_id ) {
    $query = array(
        'post_parent'       => $post_id,
        'post_type'         => 'attachment',
        'post_status'       => 'inherit',
        'post_mime_type'    => 'image',
        'numberposts'       => '-1'
        );
    $images = get_children( $query );
    return $images;
}

as a result i get an array of objects. But somehow i am unable to get the object keys transfered to a new separate array. The array of objects looks like that for example (i've left out the contained key value pairs):

Array
(
    [1074] => WP_Post Object
        (
        )

    [1067] => WP_Post Object
        (
        )
)

When i try to loop through the array of objects with foreach and write the object keys to an array i get

Catchable fatal error: Object of class WP_Post could not be converted to string

with the following snippet:

    $media = abc_attachments( $post->ID );

    foreach( $media as $key ) {
        $arrayofids .= $key . ", ";
    }
    preprint($arrayofids);

Is there a more elegant and less fatal way?