PHP

combine date and time in php

$combinedDT = date('Y-m-d H:i:s', strtotime("$date $time"));
                                            ^           ^

combine date time php

$start_y = 2020;
$start_m = 11;
$start_d = 23;

$date = $start_y . "-" . str_pad($start_m, 2, "0", STR_PAD_LEFT) . "-" . str_pad($start_d, 2, "0", STR_PAD_LEFT);