Calling sort() inside of a foreach() does not affect the original array

I promise you I've had a look at the many existing SO Qs about PHP sorting, including this mega one

I've got a PHP associative array, with strings as keys. Each value is an array of integers. I want to sort each array of integers, in simple ascending numerical order. I'm convinced this should be easy, and I've found enough examples that I think I should be doing the right thing, but it's not quite working, so there's a typo or I'm an idiot or something...

PHP:

//Each fruit corresponds to an array (series) of integers
$data = [
    'banana' => [
        1,3,2
    ],
    'orange' => [
        5,1,3
    ]
];

echo "Before sort:\n";
var_dump($data);

//For each fruit, I want to order the numbers
foreach ($data as $key => $series)
{
    //Sort array of integers
    sort($series);

    //NB I wasn't sure about value/reference details of foreach loops, so I also tried
    //retrieving a series into a variable, sorting, and then reassigning back to the same key
}

echo "\n\nAfter sort:\n";
var_dump($data);

Output:

Before sort:
array(2) {
  'banana' =>
  array(3) {
    [0] =>
    int(1)
    [1] =>
    int(3)
    [2] =>
    int(2)
  }
  'orange' =>
  array(3) {
    [0] =>
    int(5)
    [1] =>
    int(1)
    [2] =>
    int(3)
  }
}


After sort:
array(2) {
  'banana' =>
  array(3) {
    [0] =>
    int(1)
    [1] =>
    int(3)
    [2] =>
    int(2)
  }
  'orange' =>
  array(3) {
    [0] =>
    int(5)
    [1] =>
    int(1)
    [2] =>
    int(3)
  }
}

As you can see, in the output the inner arrays of integers have not been sorted. What am I doing wrong? (PHP 5.5.9, Windows 7)

Use a reference &:

foreach ($data as $key => &$series)
{
    //Sort array of integers
    sort($series);
    // OR
    // sort($data[$key]);
}