Small scope/huge frustration mystery: buggy array behavior in php - php

Question

So crazy! I have a bug that's 100% reproducible, it happens in only a few lines of code, yet I cannot for the life of me determine what the problem is.
My project is a workout maker, and the mystery involves two functions:
get_pairings: It makes a set of $together_pairs (easy) and $mixed_pairs (annoying), and combines them into $all_pairs, used to make the workout.
make_mixed_pairs: this has different logic depending on whether it's a partner vs solo workout. Both cases return a set of $mixed_pairs (in the same exact format), called by the function above.
The symptoms/clues:
The case of the solo workout is fine, $all_pairs will only contain $mixed_pairs (because as it's defined, $together_pairs are only for partner workouts)
In the case of the a partner workout, when I combine the two sets in get_pairings(), $all_pairs only successfully gets the first set I give it! (If I swap those lines at step 2 and add $together_pairs first, $all_pairs contains only those. If I do $mixed_pairs first, $all_pairs contains only that).
Then if I uncomment that second-to-last line in make_mixed_pairs() just for troubleshooting to see what happens, then $all_pairs does successfully include exercises from both sets!
That suggests the problem is something I'm doing wrong in making the arrays in make_mixed_pairs(), but I confirmed that the resulting format is identical in both cases.
Anyone see what else I could be missing? I've been narrowing down this bug for 4 hours so far- I can't make it any smaller, and I can't see what's wrong :(
Update: I updated the for loop in make_mixed_pairs() to stop at $mixed_pair_count - 1 (instead of just $mixed_pair_count), and now I sometimes get one single 'together_pair' mixed in the $all_pairs results; the same damn one each time, weirdly. Though it's not 'fixed', because again when I change the order that I add the two sets in get_pairings, when I add $together_pairs first, then $all_pairs is ENTIRELY those- it's so strange...
Here are the functions: first get_pairings (relevant part is right before and after step 2):
/**
* Used in make_workout.php: take the user's available resources, and return valid exercises
*/
function get_pairings($exercises, $count, $outdoor_partner_workout)
{
// 1. Prep our variables, and put exercises into the appropriate buckets
$mixed_exercises = array();
$together_pairs = array();
$mixed_pairs = array();
$all_pairs = array();
$selected_pairs = array();
// Sort the valid exercises: self_pairing exercises go as they are, with extra
// array for consistent formatting. Mixed ones go into $mixed_exercises array
// for more specialized pairing in make_mixed_pairs
foreach($exercises as $exercise)
{
if ($exercise['self_pairing'])
{
$pair = array($exercise);
array_push($together_pairs, [$pair]);
}
else
{
$this_exercise = array($exercise);
array_push($mixed_exercises, $this_exercise);
}
}
// Now get the mixed_pairs
$mixed_pairs = make_mixed_pairs($mixed_exercises, $outdoor_partner_workout);
// 2. combine together into one set, and select random pairs for the workout
// Add both sets to the array of all pairs (to pick from afterward)
$all_pairs += $mixed_pairs;
$all_pairs += $together_pairs;
// Now let's choose at random our desired # of pairs, and save them in $selected_pairs
$pairing_keys = array_rand($all_pairs, $count);
foreach($pairing_keys as $key)
{
array_push($selected_pairs, $all_pairs[$key]);
}
// Finally, shuffle it so we don't always see the self-pairs first
shuffle($selected_pairs);
return $selected_pairs;
}
And the other one- make_mixed_pairs: there are two cases, the first is complicated (and shows the bug) and the second is simple (and works):
/**
* Used by get_pairings: in case of a partner workout that has open space (where
* one person can travel to a point while the other does an exercise til they return)
* we'll pair exercises in a special way. (If not, fine to grab random pairs)
*/
function make_mixed_pairs($mixed_exercises, $outdoor_partner_workout)
{
$mixed_pairs = array();
// When it's an outdoor partner workout, we want to pair travelling with stationary
// put them into arrays and then we'll make pairs using one from each
if ($outdoor_partner_workout)
{
$mixed_travelling = array();
$mixed_stationary = array();
foreach($mixed_exercises as $exercise)
{
if ($exercise[0]['travelling'])
{
array_push($mixed_travelling, $exercise);
}
else
{
array_push($mixed_stationary, $exercise);
}
}
shuffle($mixed_travelling);
shuffle($mixed_stationary);
// determine the smaller set, and pair exercises that many times
$mixed_pair_count = min(count($mixed_travelling), count($mixed_stationary));
for ($i=0; $i < $mixed_pair_count; $i++)
{
$this_pair = array($mixed_travelling[$i], $mixed_stationary[$i]);
array_push($mixed_pairs, $this_pair); // problem is adding them here- we get only self_pairs
}
}
// Otherwise we can just grab pairs from mixed_exercises
else
{
// shuffle the array so it's in random order, then chunk it into pairs
shuffle($mixed_exercises);
$mixed_pairs = array_chunk($mixed_exercises, 2);
}
// $mixed_pairs = array_chunk($mixed_exercises, 2); // when I replace it with this, it works
return $mixed_pairs;
}

Answer 1

Oh for Pete's sake: I mentioned this to a friend, who told me that union is flukey in php, and that I should use array_merge instead.
I replaced these lines:
$all_pairs += $together_pairs;
$all_pairs += $mixed_pairs;
with this:
$all_pairs = array_merge($together_pairs, $mixed_pairs);
And now it all works