- var_dump: documentation ( source)
- ksort: documentation ( source)
<?php
$array = array(
"2018-10" => 239,
"2018-11" => 264,
"2018-12" => 237,
"2019-01" => 211,
"2019-04" => "0",
"2019-03" => "0",
"2019-02" => "0"
);
ksort($array);
var_dump($array);