php session_start() not working on heroku between functions - php

Question

I should start by saying that I am not super familiar with deploying apps. Most of my web development has been through a framework that handles everything for me, or it's been done locally. Now that I'm trying to deploy a personal project, I'm having issues.
I have a PHP website that I have deployed on Heroku. I have been having one issue after another with sessions (everything is working perfectly locally, but breaks on Heroku). I have solved most of the issues by going through about a million other posts. I'm using Memcachier, I'm making sure there is a favicon, I went through and added "exit();" after each header("location: ... ") call, etc.
Finally, I have sessions working almost perfectly except on one page. I have on this page the following code:
<?php
include('header.php');
include('functions.php');
//if id is set, flag as help needed
if(isset($_GET['id'])) {
flagHelped($_GET['id'], 1);
}
//if cancel is set, unflag as help needed
if(isset($_GET['cancel'])){
flagHelped($_GET['cancel'], 0);
}
//start the session and store userID in variables.
session_start();
$userID = $_SESSION['user_id'];
//grab all pets attached to this user.
$pets = getPets($userID);
?>
<!-- create table of pets -->
<div class='container bg-white'>
<table class='table'>
<tr><th>Picture</th><th>Pet Type</th><th>Pet Name</th><th>Zip Code</th><th> Profile </th></tr>
<?php
if($pets != null){
foreach($pets as $pet){
echo "<tr>";
echo "<td><img style='width:150px' src=\"".$pet["pictureLink"]."\"></td>";
echo "<td>".$pet["type"]."</td>";
echo "<td>".$pet["name"]."</td>";
echo "<td>".$pet["zipCode"]."</td>";
if($pet["needsHelp"] == 0){
echo "<td><button class = 'btn btn-success'>Request An Angel</button></td>";
}
else{
echo "<td><button class = 'btn btn-danger'>Cancel Request</button></td>";
}
echo "</tr>";
}
}
?>
</table>
<?php
include('footer.php');
?>
flagHelped() looks like this:
function flagHelped($petId, $status){
echo "in flag helped";
$connection = new mysqli(//credentials here);
$connection->query("UPDATE Pets Set `needsHelp`=".$status." where `petId`=".$petId.";");
$connection->close();
return;
}
It's a pretty simple page that prints a list of pets associated with the user that is logged in, and prints a button that redirects back to the current page as a get request with a URL variable.
The first time the pages loads (with no URL variable), there is no issue. After clicking a button, it calls the flagHelped() method, gets all the way through, saves properly to the database and returns. However, session_start() doesn't seem to do anything on return and it never gets to getPets();
I have put debugging print statements pretty much everywhere, and I have reordered the page in different ways. If I set the session before calling flagHelped(), I am able to print the $userID variable, but once I return from that function (which is in functions.php), $userID is no longer valid. Since I didn't need any of the session variables for that function, I decided I would just call the session variables after I returned. But even that isn't working.
Again, I'm sorry if I'm asking a dumb question here, I've been at it for hours and I'm at my wit's end.
Side Note: I am aware that there might be security risks on this page. This is for an application I built in less than 16 hours at a hackathon, so I'm not worried about the security risks. I'm purposely putting up how far I got before the code turn-in time. The database is filled with fake data and I have a disclaimer where I link to it saying not to use real information because of potential risks.
Edit: Adding pictures:
First time going through page (no URL variable) with echo statements
Second time going thorugh page (with URL variable) with echo statements

Answer 1

Ok, so I finally figured it out. I'm posting here in case any one else runs into this issue and comes across my post.
This worked locally and not on heroku and I couldn't figure out why. It turns out it's because I was trying to store images locally on heroku's server. When I stored them locally in my own project, they would stay in the folder that I saved them. When I stored them locally on heroku (if a user uploaded a picture on the heroku url while deployed), once my application slept after 30 minutes of inactivity, it would delete the picture.
What was happening was that my project couldn't find that image, and for whatever reason, that caused a session invalidation. Locally, all worked fine (even without being able to find the image), it only stopped working once deployed on heroku. I manually deleted those records from the database, and that page worked perfectly again.
(Now I'm looking into heroku storage options for projects to help permanently solve the problem.)