<?php

$current_time = '4:00';
$time_slot = ['1:00','3:00','15:00','19:00'];

$current_date = new \DateTime($current_time);

foreach($time_slot as &$value) {
	if ($current_date > new \DateTime($value)) $value = null;
}
$time_slot = array_filter($time_slot);

var_dump($time_slot);