<?php
$input = array
(
0 => array
(
'state' => 'CA',
'city' => 'Los Angeles',
),
1 => array
(
'state' => 'CA',
'city' => 'San Francisco',
),
2 => array
(
'state' => 'New York',
'city' => 'Brooklyn',
),
3 => array
(
'state' => 'New York',
'city' => 'Buffalo',
),
);
$output = array();
foreach ($input as $value)
{
if (array_key_exists($value['state'], $output) !== true)
{
$output[$value['state']] = array();
}
$output[$value['state']][] = $value['city'];
}
print_r($output);
?>