Php order array by number of elements

Consider this simple example array:

$letters = [
    ['a', 'c', 'f'],
    ['c', 'f'],
    ['b', 'd', 'x', 'c', 'f'],
    ['f']
];

I need to order the array in descending order by number of letters (or anyway, by the number of elements in each array item:

'b', 'd', 'x', 'c', 'f'
'a', 'c', 'f'
'c', 'f'
'f'

>Solution :

This should give you the result you are looking for:

$letters = [
    ['a', 'c', 'f'],
    ['c', 'f'],
    ['b', 'd', 'x', 'c', 'f'],
    ['f']
];

usort($letters, 'order');

function order($a, $b){
    return (count($a) < count($b) ? 1 : -1);
}

var_dump($letters);