Save multiple values from checkboxes in php

I am very new to php and teaching it to myself, so please keep that in mind.

I am working on a project that presents users with a list of items from a database with checkboxes and allows the user to check them. I want to save the values of the checked fields. This is the line of code that prints all of the options. It prints a course code and course title, with a checkbox.

echo "<input type='checkbox' name ='boxes'>" . $row['course'] . ' ' . $row['title'] . "<br>";

However, when I try to print the values selected, it doesn't work. I get an error that says "invalid argument supplied foreach()"

if(isset($_POST['submit_courses'])){
  if(!empty($_POST['boxes'])){
    foreach($_POST['boxes'] as $selected ){
      echo $selected."</br>";
    }
  }
}

Please help!