MySQL doesn't take the string value of a PHP variable (With old mysql driver)

I have code that looks like this:

    $fields = $_POST;

    $valueStrings = array();

    foreach ($fields as $key => $value) {
        array_push($valueStrings, $key . "=" . (string) $value);
        // I also tried "$key = $value"

    }

    $updateRowQuery = "UPDATE ShoppingCart
                       SET " . implode(',', $valueStrings) . "
                       WHERE cartID = $cartID";
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I get the error:

Invalid query: Unknown column 'test' in 'field list', query is: 
UPDATE ShoppingCart
SET shipToSameLocation=1,shipToSameLocation_shippingLocationID=5,shipToSameLocation_shippingMethod=test
WHERE cartID = 1405
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If I remove theshipToSameLocation_shippingMethod field, it works fine. We can see that its valuetest (other values, too) don't have quotes around despite the (string) casting in the loop.

How can I fix this?