Dynamically create an array of arrays with string key/index in PHP - php

Question

OK, this seems rather simple and I've looked for another answer, but perhaps I'm not searching for the right thing. I have a list of URLs and pages containing those URLS
https://example.com/?p=1 | https://example.com/go/test404/
https://example.com/?p=1 | https://example.com/404
https://example.com/?p=5 | https://example.com/go/test404/
https://example.com/?p=5 | https://example.com
I loop through each line and parse the URLs into $parent (the first column) and $destination (second column).
Now, I want to end up with the following, but can't figure out how to generate this structure, with the outer array called $allLinks:
Array
(
[https://example.com/go/test404/] => Array
(
[0] => https://example.com/?p=1
[1] => https://example.com/?p=5
)
[https://example.com/404] => Array
(
[0] => https://example.com/?p=1
)
[https://example.com] => Array
(
[0] => https://example.com/?p=5
)
)
Thank you very much for assistance in this.
In a loop, I've tried $allLinks[$destination] .= [$parent] and array_push($allLinks[$destination], $parent) but neither seem to be working for me.

Answer 1

You can do this as you are reading/separating the parts. I don't know how you're doing that, but the below assumes you have read lines from a file into an array. Just use the parent as the key and append [] each destination to the array:
foreach($file as $line) {
list($parent, $destination) = explode(' | ', $line);
$result[$parent][] = $destination;
}
That is how I would do it, but rereading it seems you want the opposite:
foreach($file as $line) {
list($parent, $destination) = explode(' | ', $line);
$result[$destination][] = $parent;
}
array_push requires an array as the first argument, so to use it (not recommended) you'd have to check if it is set and if not then define it as an array first. Something like:
$result[$destination] = $result[$destination] ?? [];
array_push($result[$destination], $parent);
The top line translates into:
if(!isset($result[$destination])) {
$result[$destination] = [];
}