Returning nested array with added key=>value

Question

This works, but do I have to make a new array, result, or is there a way to just insert the field I need into each $key of the received parameter $data?

Won't this way take up more memory needlessly?

function add_tax($data){
            $result = [];
            foreach($data as $product => $details) {
                $details['tax'] = 0;
                $result[$product] = $details;
                } 
            return $result;
          }

Answer 1

You could try something along these lines:

function add_tax($data){
        foreach($data as $product => $details) {
            $data[$product]['tax'] = 0;
        }
        return $data;
}

EDIT 1:

After a bit more researching on foreach I have come up with the following alternative:

function add_tax(&$data) {
    foreach($data as &$details)
        $details['tax'] = 0;
}

In reference to Alex L: I apologize, the original answer was rushed. The first snippet was simply a way of answering the question, that yes, it is possible to iterate through $data without generating a new variable. Also, to MikeiLL, yes, that would be a waste of memory. This second snippet passes both variables by reference, which does not require any excess usage of memory. It is the most optimal format of this process.

NOTE: The unset in the function is not exactly necessary, but as your question referenced memory usage, unset breaks the reference and releases that memory.

EDIT 2:

Due to very strong backing in the comments, I've removed the call to unset (truth be told I'd never use it myself, but wanted to reference it's usage within the function)

Answer 2

PHP provides quite a few handy array-manipulation functions. This is a case for array_map which passes each value to a callback and assigns it to the same key in a new array.

function add_tax(array $data) {
    return array_map(function ($details) {
        $details['tax'] = 0;
        return $details;
    }, $data);
}

This version won't modify the original $data array passed to add_tax.

How does this differ from writing your own loop using foreach? Mostly in style. You'll pay a minor performance penalty switching between system and user space (once for each call to the callback), but you won't notice this unless $data is huge.

It looks much nicer when the callback returns an immediate expression instead of using one statement to modify and another to return. You can do this here using array_merge. In fact, this also allows you to refactor add_tax into a generic helper function.

function extend_arrays(array $arrays, array $additions) {
    return array_map(function ($array) use ($additions) {
        return array_merge($array, $additions);
    }, $arrays);
}

function add_tax($data) {
    return extend_arrays($data, array('tax' => 0));
}

function add_tax_and_shipping($data) {
    return extend_arrays($data, array('tax' => 0, 'shipping' => 0));
}