PHP Comparing two multidimensional arrays with foreach - php

Question

I'm attempting to compare non-matching values within two multidimensional arrays ($allSessions, my master array and $userSessions, my inner array...everything in it should be within $allSessions, but structured differently) and my approach was to use a foreach within a foreach loop.
This works under most situations except one (when $userSession only contains one item).
I'm wondering if the bug is caused by this loop within a loop? When it is buggy because $userSessions only contains 1 item, the returned $unregistered array contains multiples of each item...
$allSessions = $this->getAllUpcoming();
$unregistered = array();
$userSessions = $this->getUserSessions($userID);
foreach ($allSessions as $session) {
foreach ($userSessions as $user) {
if ($user["entry_data"]["session-participant-session"]["id"] !== $session["id"]){
array_push($unregistered, $session);
}
}
}

Answer 1

In the way you have it, you will get every non-matching element.
Let's say you have a perfect match of a,b,c in $allSessions and a,b,c in $userSessions. In your first outer loop you have 'a'. In the inner loop you will add 'b' and 'c' to your $unregistered because they don't match. Then you go on to 'b' in your outer loop and add 'a' and another copy of 'c' in your inner loop. And so on.
I'm pretty you're going to have to structure it differently. You've got to check every element in $userSessions and go on to the next element in $allSessions only if you don't find any matches:
foreach ($allSessions as $session) {
foreach ($userSessions as $user) {
if ($user["entry_data"]["session-participant-session"]["id"] === $session["id"])
continue 2; // this goes to the next element in $allSessions
}
array_push($unregistered, $session);
}
A slightly more readable form if you are not familiar with continue:
foreach ($allSessions as $session) {
$found = false;
foreach ($userSessions as $user) {
if ($user["entry_data"]["session-participant-session"]["id"] === $session["id"]) {
$found = true;
break; // an optimization - not strictly necessary
}
}
if (!$found)
array_push($unregistered, $session);
}