array_replace() / array_merge() | ( $_SESSION = array() ) argument is not an array?

I have this code for my school project and thought the code does its job on what i wanted it to do, i still keep on getting the error about $_SESSION[] is not an array argument when using the array_replace() and array_merge() functions:

Session is already initiated on the header:

 // Start Session
 session_start();

For initialising the $_SESSION['cart'] as an array:

// Parent array of all items, initialized if not already...
if (!isset($_SESSION['cart'])) {
    $_SESSION['cart'] = array();
}

For adding products from a dropdown menu: - (Just to see how the session is assigned:)

if (isset($_POST['new_item'])) { // If user submitted a product
 $name = $_POST['products']; // product value is set to $name

// Validate adding products:
if ($name == 'Select a product') { // Check if default - No product selected
    $order_error =  '<div class="center"><label class="error">Please select a product</label></div>';
} elseif (in_array_find($name, $_SESSION['cart']) == true) { // Check if product is already in cart:
    $order_error = '<div class="center"><label class="error">This item has already been added!</label></div>';
} else {
    // Put values into session:
    // Default quantity = 1:
    $_SESSION['cart'][$name] = array('quantity' => 1);
       }
}

Then here is where the issue comes, when they try to update the product:

// for updating product quantity:
if(isset($_POST['update'])) {
// identify which product to update:
$to_update = $_POST['hidden'];
// check if product array exist:
if (in_array_find($to_update, $_SESSION['cart'])) {
    // Replace/Update the values:
    // ['cart'] is the session name
    // ['$to_update'] is the name of the product
    // [0] represesents quantity
    $base = $_SESSION['cart'][$to_update]['quantity'] ;
    $replacement = $_SESSION['cart'][$to_update] = array('quantity' => $_POST['option']);
    array_replace($base, $replacement);
    // Alternatively use array merge for php < 5.3
    // array_merge($replacement, $base);
    }
}

Note that both the functions array_replace() and array_merge() are updating the values and doing what the initial goal was, but the problem is that i still keep on getting that one argument($base) is not an array issue.

Warning: array_replace() [function.array-replace]: Argument #1 is not an array in ...

any suggestions for a better way to approach this issue would be a valuable help :) Thanks for your help :)

Edit: The in_array_find() is a function that i use in replacement of in_array() as it doesn't apply to finding values in a multi-dimensional array: specifically 2 depth arrays:

found it from here and it works for me

The code for it is:

// Function for searching values inside a multi array:
function in_array_find($needle, $haystack, $strict = false) {
foreach ($haystack as $item => $arr) {
    if (($strict ? $item === $needle : $item == $needle) || (is_array($item) && in_array_r($needle, $item, $strict))) {
        return true;
    }
}

return false;
}