Fetch values from one column of a table with PDO

I'm trying to fetch values from one table column of a MySQL db with PHP (PDO), but I have two errors in the same line, this is the line (line 7):

$query = $dbh->prepare("SELECT * FROM table_name");

And these are error messages:

Notice: Undefined variable: dbh in ... on line 7

Fatal error: Call to a member function prepare() on a non-object in ... on line 7

My complete code:

1) file config.php

    <?php
$host = "localhost";

$db_user = "user";

$db_psw = "pass";

$db_name = "database";
?>

2) file connection.php

<?php

       include("config.php");

        //collegamento
    $col = "mysql:host=$host;dbname=$db_name";

        try {
                  //tentativo di connessione
          $db = new PDO($col , "$db_user", "$db_psw");
        }
                    //gestione errori
            catch(PDOException $e) {

              echo 'Attenzione errore: '.$e->getMessage();
            }      

?>

3) file display-values.php (error in line 7)

<?php
// richiamo lo script responsabile della connessione a MySQL
require 'connection.php';
?>

<?php
$query = $dbh->prepare("SELECT * FROM table_name");
$query->execute();
$result = $query->fetchall();
?>

<?php
    echo 
    "<table border='2'>
    <tr>
    <th>ID</th>
    <th>A Number</th>
    <th>First Name</th>
    <th>Last Name</th>
    <th>Why</th>
    <th>Comments</th>
    <th>Signintime</th>
    </tr>"
    ;

    foreach($result as $row)
    {
  echo "<tr>";
  echo "<td>" . $row['id'] . "</td>";
  echo "<td><a href=Student.php?studentA_num=" . $row['anum'] . ">" .$row['anum'] . " </a></td>";
  echo "<td>" . $row['first'] . "</td>";
  echo "<td>" . $row['last'] . "</td>";
  echo "<td>" . $row['why'] . "</td>";  
  echo "<td>" . $row['comments'] . "</td>";
  echo "<td>" . $row['signintime'] . "</td>";
  echo "<td> <input type=\"button\" value=\"Start Session\"onClick=\accept.php?id=" . $row['id'] . "&start=true></td>";
}

  echo "</tr>";
  echo "</table>";
?>

Note: In this code I've tried to fetch all values (just for a test), but I need to fetch data from one column "name_column" only, so how can I fix the two errors and edit the code to extract values from a specific column?

Thanks in advance!