$hour_list = range(0,24);

foreach($hour_list as $key=>$val){

$hour_list[$key] = str_pad($val, 2, "0", STR_PAD_LEFT);

}

var_dump($hour_list);

$hour_list = range(0,24);

foreach($hour_list as $key=>$val){

$hour_list[$key] = sprintf('%02d',$val);

}

var_dump($hour_list);