I am using the Google Sheet API. My code is working as expected. Now though, I want to add some bolding, highlighting, etc. I have reviewed other pieces of code but I am not fully understanding how to do this. Thank you!!

$service = new Google_Service_Sheets($client);
$spreadsheetId = 'xxxxxxx';
$spreadsheet = $service -> spreadsheets -> get($spreadsheetId);
$result = $db -> query("SELECT * from xxxxxx");
$oldref = '1';
if ($result -> num_rows > 0) {
  $rows = array();
  $i = 0;
  while ($row = $result -> fetch_assoc()) {
    $i++;
    if ($oldref != $row['refnumber']) {
  ///////////////////////////////////////////////////
 ////this is where I want to BOLD the $row['refnumber'] value.//////       
 $newRow = [$row['refnumber']];
  array_push($rows, $newRow);       
    }
  
  }
  $valueRange = new Google_Service_Sheets_ValueRange();
  $valueRange -> setValues($rows);
  $range = 'Sheet1';
  $options = ['valueInputOption' => 'USER_ENTERED'];
  $service -> spreadsheets_values -> update($spreadsheetId, $range, $valueRange, $options);

}