<?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  $index => $value) {
	if ($current_date > new \DateTime($value)) unset($time_slot[$index]);
}
var_dump($time_slot);