PHP variable reverts back to last assigned after intensive curl operation - memory

Question

I'm querying one api and sending data to another. I'm also querying a mysql database. And doing all this about 40 times in one second. Then waiting a minute and repeating. I have a feeling I'm at the limit of what PHP can do.
My question is about two variables that will randomly revert back to their last value, from the previous loop. They only change their value after the call to self::apiCall() (below in the second function). Both $product and $productId will randomly change their value, about once every 40 loops or so.
I boosted PHP to 7.2, increased memory to 512, and assigned some variables to null to save memory. I'm not getting any official memory warnings, but watching the variables randomly go back to their last value is perplexing. Here's what the code looks like.
/**
* The initial create products loop which calls the secondary function where
* the variables can change.
**/
public static function createProducts() {
// Create connection
$conn = new mysqli(SERVERNAME, USERNAME, PASSWORD, DBNAME, PORT);
// Check connection
if ($conn->connect_error) {
die("Connection failed: " . $conn->connect_error);
}
// This will go through each row and echo the id column
$productResults = mysqli_query($conn, "SELECT * FROM product_creation_queue");
if(mysqli_num_rows($productResults) > 0) {
$rowIndex = 0;
while($row = mysqli_fetch_assoc($productResults)){
self::createProduct($conn, $product);
}
}
}
/**
* The second function where I see both $product and $productId changing
* from time to time, which completely breaks the code. Their values
* only change after the call to self::createProduct() which is simply a
* curl function to hit an api endpoint.
**/
public static function createProduct($mysqlConnection, $product) {
// convert back to array from json
$productArray = json_decode($product, TRUE);
// here the value of $productId is one thing
$productId = $productArray['product']['id'];
// here is the curl call
$addProduct = self::api_call(TOKEN, SHOP, ENDPOINT, $product, 'POST');
// and randomly here it can revert to it's last value in a previous loop
echo $productId;
}

Answer 1

The problem was that the entire 40-query procedure took more than one minute to complete. And the cron job that started the procedure on the minute would start the next one before the first one had completed, thereby somehow re-assigning variables on the fly. The queries usually took less than one minute, but when it was longer, the conflicts appeared, thus leading to the appearance of randomness.
I reduced the number of queries per minute so now the process completes in less than 60 seconds and no variables are ever overwritten. I still don't understand how the variables would change if two php processes are happening at the same time--it seems like they would be siloed.