How to insert multiple array values into database php

Please I need your help, I am new to PHP and I need your help in inserting multiple select array into the database in the form of this.
This is what I want to achieve. My table has 5 columns (id, examno, subjects, grades, results). I want results column to be inform of subjects grades, subjects grades.........depending on the numbers of subjects and grades users select (e.g English C6, Mathematics C6) all in one column results.

This is my html codes

<form action="insert.php">
<div class="form-group">
<label>Exam Number</label> 
<input type="text" class="form-control" name="examno" id="examno">
</div>
<table width="100%" cellpadding="0" cellspacing="0" border="0" class="table table-borderless" id="example">
               <tr>  
                        <td width="12%"><label class="control-label">S/NO</label></td> 
                        <td width="53%"><label class="control-label">SUBJECTS</label></td> 
                        <td width="35%"><label class="control-label">GRADE</label></td>
               </tr> 
               <tr> 
                          <td>1</td> 
                          <td> 
                                   <select name="subjects[]" class="form-control" id="subject">
                                      <option value="" selected="selected">Select subject</option> 
                                      <option value="English">English</option> <option value="Mathematics">Mathematics</option>
                                  </select> 
                           </td> 
                           <td> 
                                     <select name="grades[]" class="form-control"> 
                                     <option value=""> Select</option> 
                                     <option value="A1">A1</option> 
                                     <option value="B2">B2</option> 
                                </select> 
                                </td>
                                </tr> 
                                </table> 
                                <input type="submit">
                                </form>

I have about 8 subjects and grades to be inserted like that

Also this is my insert.php

if(isset($_POST['submit'])){
$examno = mysqli_real_escape_string($conn, $_POST['examno']);
foreach($_POST['subjects'] as $row=>$subjects){
        $subjects = mysqli_real_escape_string($conn, $subjects);
        $grades = mysqli_real_escape_string($conn, $_POST['grades'][$row]); 
        $results = $subjects." ".$grades;
    }
    $sql = "INSERT INTO qualifications(examno, subjects, grades, results) VALUES('".$examno."', '".$subjects."', '".$grades."', '".$results."')";
    $result = mysqli_query($conn, $sql);
    if($result){
         header("location:declaration.php"); // user will be taken to the success page
            }
            else{
               echo "Oops. Something went wrong. Please try again"; 
            }
}

Nothing was inserted into the subjects, grades, and results.

Please help me as I am new to php/mysqli

How to insert multiple data into db in php mysql?

How can we store multidimensional array in database using PHP?

What is $row in PHP?