new array not appearing after foreach - php

Question

I have the following code, as you can see I would like to create a new array inside the foreach. Even though its adding perfectly fine WITHIN the loop , all seems to be forgotten once the loop is finished.
foreach ($results as $result) {
$result['categories'] = array();
echo '<pre>';print_r($result);echo '</pre>';
}
echo '<pre>';print_r($results);echo '</pre>';
Result of first print_r
Array
(
[word_two_id] => 2
[categories] => Array
(
)
)
Array
(
[word_two_id] => 3
[categories] => Array
(
)
)
Array
(
[word_two_id] => 5
[categories] => Array
(
)
)
Array
(
[word_two_id] => 12
[categories] => Array
(
)
)
Result of second print_r
Array
(
[0] => Array
(
[word_two_id] => 2
)
[1] => Array
(
[word_two_id] => 3
)
[2] => Array
(
[word_two_id] => 5
)
[3] => Array
(
[word_two_id] => 12
)
)

Answer 1

$result in the foreach is going to be overwritten in your loop on every iteration. e.g. every time the loop rolls around, a NEW $result is created, destroying any modifications you'd done in the previous iteration.
You need to refer to the original array instead:
foreach ($results as $key => $result) {
^^^^^^^
$results[$key]['categories'] = array();
^^^^^^^
Note the modifications. You may be tempted to use something like
foreach($results as &$result)
^---
which would have worked, but also leave $result a reference pointing somewhere inside your $results array. Re-using $result for other purposes later on in the code would then be fiddling with your array, leading to very-hard-to-track bugs.

Answer 2

In PHP, the foreach loop operates on a shallow copy of the array, meaning that changes to the elements of the array won't propagate outside of that loop.
To pass the array elements by reference instead of by value, you put an ampersand (&) before the name of the element variable, like so:
foreach ($results as &$result) {
$result['categories'] = array();
echo '<pre>';print_r($result);echo '</pre>';
}
This way, any changes to the array elements are instead performed on a reference to that element in the original array.
Marc B made a good point in his answer regarding a consequence of using this method. After the foreach loop is done and the code continues, the variable $result will continue to exist as a reference to the last element in the array. So, you shouldn't reuse the $result variable without removing its reference first:
unset($result);

Answer 3

You will need the key too
try this
foreach ($results as $key=>$result) {
$result['categories'] = array();
$results[$key] = $result;
}
echo '<pre>';print_r($results);echo '</pre>';