Submit multiple checkbox together with textbox input value using PHP and MySQL?

I'm making online test/quiz for students and using PHP and MySQL. Currently I have a form, where I can add the questions and answers into the database.

However, I have trouble with submitting correct answers into the database by selecting multiple checkboxes. I want to store all correct answers in the table called correct_answer(id, question_id, answer_id) and insert all selected id's from the table called answers(id,answer_id, answer).

How can I insert only selected checkboxes values and skip somehow unchecked checkboxes?

My HTML part:

<form id="add_question_form" action="add.php" method="post" enctype="multipart/form-data">
  <label>Question: *</label>
  <input id="question" type="text" name="question" class="form_default" placeholder="Type your question here" value="<?= isset($_POST['question']) ? $_POST['question'] : '' ?>" required>
  <div class="inputs">
    <label>Answer #1: *</label>
    <input type="text" id="answer" name="dynamic[]" class="form_default" placeholder="answer" value="<?= isset($_POST['dynamic'][0]) ? $_POST['dynamic'][0] : '' ?>" required/>
    <input type="checkbox" name="selected_item[]" id="chechbox" value="<?php echo $answer_id;?>">
    <label>correct</label>
    <br>
    <label>Answer #2: *</label>
    <input type="text" id="answer" name="dynamic[]" class="form_default" placeholder="answer" value="<?= isset($_POST['dynamic'][1]) ? $_POST['dynamic'][2] : '' ?>" required/>
    <input type="checkbox" name="selected_item[]" id="chechbox" value="<?php echo $answer_id;?>" />
    <label>correct</label>
    <br>
  </div>
  <input id="submit" type="submit" name="add_question" value="Submit" class="button">
</form>

PHP code:

if (isset($_POST['add_question'])) { // Checks if the form has been submitted
        if (isset($_POST['selected_item']) && !empty($_POST['selected_item'])) {
                $counter++; // each time adds 1 after user posts the question

$optionArray = $_POST['dynamic']; // submits multiple answers to the database

  for ($j = 0; $j < count($optionArray); $j++) {
                                                
        $answer = mysql_real_escape_string($_POST['dynamic'][$j]); // returns multiple answers
                                        
        $check_answer = "SELECT id, answer FROM answers WHERE answer = '$answer'";
        $check_answer = mysql_query($check_answer); //checks if answer is exist in the database
                                        
        while ($row = mysql_fetch_assoc($check_answer)) { // fetch a result rows as an associative array
                $answer_id = $row['id']; // answer id from database
        }
                                        
        if (mysql_num_rows($check_answer) > 0) { 
                $q = "INSERT INTO correct_answer (question_id, answer_id) VALUES ('$counter','$answer_id')";
                $q = mysql_query($q); 
        }
  }
}
}

Example:

1. 
   

   I'm adding question and two answers "a" and "b". Selecting "b" as correct answer.

   
   


2. 
   

   All answers have been successfully added to MySQL "answers" table.

   
   


3. 
   

   But, instead of adding id ="2" (as "b" in answers got id=2") into "correct_answer" table, it adds all answers 1 and 2..

   
   

Can anyone help me please? I also want to insert multiple correct answers, but only if they are selected..

Thanks in advance, any help will be appreciated.