PHP变量包含循环[关闭]

Here's the scenario, I have a form that contains checkboxes and when submitted will be sent to a certain email. I am trying to put the checked items into one string. Like option1 option2 option 3

Here is what I have done for now:

echo $message = foreach($_POST['checkbox'] as $checkbox){ echo $checkbox . "
";}

Desired output of $message:

Option1
Option2
Option3

Please make it as:

$message = '';
if (! empty($_POST['checkbox'])) {
  foreach($_POST['checkbox'] as $checkbox) {
    $message .= $checkbox . "
";
  }
}