$array = [
    '15',
    '20',
    '25',
    '30',
    '35',
    '40'
];

foreach($array as $key => $value){
    if($key == 0){
        echo $array[$key] . '<br>';
    }
    else{
        echo ($array[$key] - $array[$key-1]) . '<br>';
    }
}