$thehour = date('G');
 
foreach (range(0, 7) as $number) {
    $morning[$number] = $number;
}
foreach (range(8, 15) as $number) {
    $day[$number] = $number;
}
foreach (range(16, 23) as $number) {
    $evening[$number] = $number;
}
 
if(in_array($thehour,$morning)) { echo 'it\'s morning'; } // 00:00 - 07:59am
elseif(in_array($thehour,$day)) { echo 'it\'s day'; } // 08:00 - 15:59pm
elseif(in_array($thehour,$evening)) { echo 'it\'s evening'; } // 16:00pm - 23:59pm