<?php
$arr=[
[
'date' => '2019-12-29',
'event' => 'name1'
],
[
'date' => '2019-12-31',
'event' => 'name2'
],
[
'date' => '2019-12-29',
'event' => 'name3'
],
[
'date' => '2019-12-30',
'event' => 'name4'
],
[
'date' => '2019-12-29',
'event' => 'name5'
],
[
'date' => '2019-12-31',
'event' => 'name6'
],
[
'date' => '2019-12-29',
'event' => 'name7'
],
[
'date' => '2019-12-30',
'event' => 'name8'
],
[
'date' => '2019-12-30',
'event' => 'name9'
],
];
foreach ($arr as $value) {
$date[($value['date'])][]=$value['event'];
}
ksort($date);
print_r($date);