在记录结果中插入0作为最后一个ID

Question

出于某种原因，当我运行我的SQL查询时，它为Last ID返回0。我的插入查询不起作用，但它在运行时没有给我任何错误。我检查数据库，没有插入或添加任何新内容。奇怪的是，这个查询实际上在另一个页面上工作，我在这里逐字复制，但它不起作用或插入。我很难过这个。我无法弄清楚为什么当我运行此查询时它不会执行插入操作。

require 'database.php';

$ingredient = (string)$_POST['ingredient'];
$meal = (string)$_POST['meal'];

if(isset($ingredient) && ($meal)){
    $ingr_exploded = explode(" @", $ingredient);
    $ingr = $ingr_exploded[0];
    $store = $ingr_exploded[1];

// copy ingredients columns into meal_ingredients table.
$sql = "INSERT INTO meal_ingredients (ingredients_id_fk, quanity, package, cost, store)
        SELECT ingredient_id, quanity, package, cost, store
        FROM ingredients
        WHERE item='$ingr' AND store='$store'";

    if ($conn->query($sql) === TRUE) {$last_id = $conn->insert_id;
    echo "Last ID = $last_id</br>";}    
    else {echo "Error: " . $sql . "<br>"  . $conn->error;}

// insert meal_name & ingredient_name into table.
$sql2 = "UPDATE meal_ingredients
        SET meal_name='$meal', item='$ingr'
        WHERE meal_ingredients_id=$last_id";

    if ($conn->query($sql2) === TRUE) {echo "Meal Name - Ingredient Set. </br>";}   
    else {echo "Error: " . $sql . "<br>"  . $conn->error;}

}