$array[$key] = $something creates a new array - php

Question

I have this php code that's supposed to check how many times a single item is in an array and put the amount in the value of that key.
im asking, why does the $duplicates[$item] += 1; create a new array instead of appending it to the existing array
here is the picture of the output I get.
this is my code:
$itemQuery = $con->prepare("SELECT cart_value FROM active_carts WHERE username=:prodName");
$itemQuery->bindParam(":prodName" , $uname , PDO::PARAM_STR);
$itemQuery->execute();
$itemCount = $itemQuery->fetchAll();
$arrax = $itemCount[0]["cart_value"];
$itemArrX = explode(",", $arrax);
$inQuestion = array();
$duplicates = array();
foreach ($itemArrX as $item) {
if (in_array($item , $inQuestion)) {
$counter = 0;
if (!array_key_exists($item , $duplicates)) {
$duplicates[$item] = $counter;
// doesnt even execute
} else {
echo $duplicates[$item]; // echoes a new array every time
$duplicates[$item] += 1;
}
} else {
array_push($inQuestion, $item);
}
}

Answer 1

It is rather simple, you never created that item in the array.
$data[$key] = $value; //examine that.
try this $duplicates[$item] = $item;
then $duplicates[$item] += 1;

Answer 2

If you're just looking for a count of duplicate values, I like the suggestion about array_count_values() in the comments. That can be run through array_filter() to remove the non-duplicate values.
$itemQuery = $con->prepare("SELECT cart_value FROM active_carts WHERE username=:prodName");
$itemQuery->execute([":prodName"=>$uname]);
$itemArrX = explode(",", $itemQuery->fetchColumn(0));
$itemArrX = ["foo","bar","baz","bar","boo","baz"];
$duplicates = array_filter(array_count_values($itemArrX), function($v){return $v>1;});
$inQuestion = array_unique($itemArrX);
print_r($duplicates);
print_r($inQuestion);
Or here's a condensed version of your code which should work just fine.
$itemQuery = $con->prepare("SELECT cart_value FROM active_carts WHERE username=:prodName");
$itemQuery->execute([":prodName"=>$uname]);
$itemArrX = explode(",", $itemQuery->fetchColumn(0));
$itemArrX = ["foo","bar","baz","bar","boo","baz"];
$duplicates = [];
$inQuestion = [];
foreach ($itemArrX as $item) {
if (in_array($item, $inQuestion)) {
echo $duplicates[$item];
$duplicates[$item]++;
} else {
$duplicates[$item] = 1;
$inQuestion[] = $item;
}
}
print_r($duplicates);
print_r($inQuestion);