<?php

$data = array(
	array("date" => "2018-01-21", "count" => 1, "peopleCount"=> 2),
	array("date" => "2018-01-22", "count" => 1, "peopleCount"=> 3),
	array("date" => "2018-01-19", "count" => 1, "peopleCount"=> 4),
	array("date" => "2018-01-25", "count" => 1, "peopleCount"=> 5)
	);
	

usort($data, function($d1, $d2){
	$t1 = strtotime($d1['date']);
	$t2 = strtotime($d2['date']);
	if ($t1 === $t2) return 0;
	return ($t1 > $t2) ? -1 : 1;
});

var_dump($data);