SQL connection dies after one call - php

Question

I have a custom class that takes a sql connection as a parameter. I use that to populate the class, and then I'm trying to use it again to modify the results on screen. But after the first use, I can't use it anymore.
connection.php:
$conn = new mysqli('localhost', 'root', '', 'loveConnections');
if ($conn->connect_error) {
die("Connection failed: " . $conn->connect_error);
}
personalityProfile.php (front end)
if (!isset($_SESSION['interests'])) {
$interests = new Interests($conn, $_SESSION['id']);
$_SESSION['interests'] = $interests;
} else {
$interests = $_SESSION['interests'];
}
interestsObject.php
class Interests {
// properties
public $conn;
public $id;
public $interestsArray = [];
public function __construct($conn, $memberId = null, $intArray = [
'basketball' => false,
'bowling' => false,
'movies' => false,
]) {
$this->conn = $conn;
$this->id = $memberId;
$this->interestsArray = $intArray;
$this->popArraySql();
}
public function popArraySql() {
$memInterests = [];
$sql = "SELECT i.interest
FROM memberInfo m
Join MemberInterestLink mi on (mi.memberID_FK = m.memberID_PK)
Join interests i on (mi.interestID_FK = i.interestID_PK)
WHERE memberID_PK = $this->id";
$result = $this->conn->query($sql);
$this->conn works perfectly here
foreach ($result as $row) {
array_push($memInterests, $row['interest']);
}
foreach ($this->interestsArray as $key => $value) {
for ($i=0; $i<sizeof($memInterests); $i++) {
if ($memInterests[$i] === $key) {
$this->interestsArray[$key] = true;
}
}
}
}
public function insertUpdateQuery() {
var_dump($this->conn);
foreach ($this->interestsArray as $key => $val) {
echo $key . "<br>";
$select = "SELECT interestID_PK from interests where interest = '" . $key . "'";
echo $select;
$result = $this->conn->query($select);
when I try and use it later though, I get a Warning: mysqli::query(): Couldn't fetch mysqli. Additionally, if I try and var_dump it, I get Warning: var_dump(): Property access is not allowed yet
var_dump($result);
if ($val === true) {
$insert = "INSERT INTO MemberInterestLink (memberID_FK, interestID_FK) VALUES ($this->id, $interestKey)";
$this->conn->query($insert);
} else {
$delete = "DELETE FROM MemberInterestLink WHERE interestID_FK = $interestKey";
$this->conn->query($delete);
}
}
}
}
I never close the connection, which is what most of the related answers suggested the cause may be. It's like my $conn variable just stops working after the first use.