Вот мой простой подход.
$values = array();
foreach($workouts_id as $value){
$_value = "(".$value.",".$plan_id.")";
array_push($values,$_value);
}
$values_ = implode(",",$values);
$sql = "INSERT INTO plan_days(id,name) VALUES" . $values_."";
$stmt = $this->conn->prepare($sql);
$stmt->execute();