Get Posts by IDs (optionally)

It looks like WordPress doesn’t ignore empty post__in param. So you’ll have to do this yourself like this…

$ids="1,2,3";

if ( $ids ) {
    $arr = explode("," ,$ids);

    $args = array(
        'posts_per_page' => $number,    
        'post__in' => $arr
    );
} else {
    $args = array(
        'posts_per_page' => $number
    );
}