PHP: 二维数组排序

gistfile1.txt

待排数组：
```
$user = [
  [
    'name' => 'tom',
    'age' => 10,
  ],
  [
    'name' => 'Jim',
    'age' => 9,
  ],
  [
    'name' => 'Janne',
    'age' => 18,
  ]
]
```

usort ($user, function($a, $b) {
   return $a.age - $b.age;
})