I need help entering data into database table with PDO

Hi, I'm trying to enter data into database & get these messages:
Warning: Illegal string offset 'purpose' in C:\xampp\htdocs\testing\pdocalc.php on line 23

Warning: Illegal string offset 'number1' in C:\xampp\htdocs\testing\pdocalc.php on line 23

Warning: Illegal string offset 'opvalue' in C:\xampp\htdocs\testing\pdocalc.php on line 23

Warning: Illegal string offset 'number2' in C:\xampp\htdocs\testing\pdocalc.php on line 23

Warning: Illegal string offset 'total' in C:\xampp\htdocs\testing\pdocalc.php on line 23

Fatal error: Call to a member function prepare() on a non-object in C:\xampp\htdocs\testing\pdocalc.php on line 24
After a lot of research I haven't resolved this.

<?php 
       function _NewDB()
 {
       try  
 {
 $databaseName = "root";$databaseUser = "homedb";$databasePass = "cookie";
 $dbh = new PDO('mysql:host=localhost;dbname='.$databaseName,      $databaseUser, $databasePass);
         $dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
 } 
catch (PDOException $e)
          {error_log("PDOException: " . $e);return false;}
    return $dbh;
 }
   // End connection stuff            
  $dbh = _NewDB();
  $fieldList = array("purpose", "number1", "opvalue", "number2", "total" );             
  if(!$_POST) exit("No data submitted via POST!");
      $data = json_encode($_POST, true);                
       foreach($fieldList as $field) 
 if(!$data[$field]) exit("No data submitted for '" . $field . "'!");     
            $stmt = $dbh->prepare("INSERT INTO calculator (purpose, number1, opvalue, number2, total) VALUES(:purpose, :number1, :opvalue, :number2, :total)");
            $stmt->execute($_POST);                     // line 22
            echo 'Sucessfully added a new row to the calculator table!';
    // Below pulling rows/results from the table with PDO            
            //$stmt = $dbh->prepare("SELECT * FROM calculator");
            //$stmt->execute();
            //$rows = $stmt->fetchAll(PDO::FETCH_ASSOC);
header( "refresh:3;url='http://localhost/testing/pdocalc.html'");
?>