<form enctype="multipart/form-data" method="post">
<input type="text" class="form-control" name="student_name" placeholder="Name Student" Value="David" required><br/>
<select class="form-control" name="course_id[]" multiple required>
<option value="1" selected>Photoshop</option>
<option value="2">DreamWeaver</option>
<option value="3" selected>Idesign</option>
<option value="4">Illustrator</option>
</select><br/> <button type="submit">Submit</button> </form>

<form enctype="multipart/form-data" method="post">
<input type="text" class="form-control" name="student_name" placeholder="Name Student" Value="David" required><br/>
<select class="form-control" name="course_id[]" multiple required>
<option value="1" selected>Photoshop</option>
<option value="2">DreamWeaver</option>
<option value="3" selected>Idesign</option>
<option value="4">Illustrator</option>
</select><br/> <button type="submit">Submit</button> </form>

<form enctype="multipart/form-data" method="post">
<input type="text" class="form-control" name="student_name" placeholder="Name Student" Value="David" required><br/>
<select class="form-control" name="course_id[]" multiple required>
<option value="1" selected>Photoshop</option>
<option value="2">DreamWeaver</option>
<option value="3" selected>Idesign</option>
<option value="4">Illustrator</option>
</select><br/> <button type="submit">Submit</button> </form>

<form enctype="multipart/form-data" method="post">
<input type="text" class="form-control" name="student_name" placeholder="Name Student" Value="David" required><br/>
<select class="form-control" name="course_id[]" multiple required>
<option value="1" selected>Photoshop</option>
<option value="2">DreamWeaver</option>
<option value="3" selected>Idesign</option>
<option value="4">Illustrator</option>
</select><br/> <button type="submit">Submit</button> </form>

<form enctype="multipart/form-data" method="POST" action="php/update.php">
  <select name="student_name">
    <option value="1">Ana</option>
    <option value="2">David</option>
    <option value="3">Jasmine</option>
</select>

  <select class="form-control" name="course_id[]" multiple required>
    <option value="1" selected>Photoshop</option>
    <option value="2">DreamWeaver</option>
    <option value="3" selected>Idesign</option>
    <option value="4">Illustrator</option>
</select>
    <br/>
    <button type="submit">Submit</button>
</form>

<?php
$servername = "localhost";
$username = "username";
$password = "password";
if(isset($_POST["student_name"])){$student_name = $_POST["student_name"];}else{$student_name = "";}
if(isset($_POST["course_id"])){$course_id = $_POST["course_id"];}else{$course_id = "";}

try
{
  $conn = new PDO("mysql:host=$servername;dbname=DB", $username, $password);
  $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

  for ($i=0; $i < count($course_id); $i++)
  {
      $sql = "UPDATE STUDENT_COURSES SET STUDENT_ID = $student_name WHERE COURSE_ID = ".$course_id[$i];
      $conn->exec($sql);
  }
}
catch(PDOException $e)
{
  echo "Conexión fallida: " . $e->getMessage();
}
 ?>