Ksort

$array = [
'2017-09-07' => [],
'2017-09-01' => [],
'2017-09-03' => [],
 
 
$array = ksort($array); // true

<?php
$array = [
'2017-09-07' => [],
'2017-09-01' => [],
'2017-09-03' => [],
 ];
 
ksort($array);
 
print_r($array);