need help for a simple query

$currentID = $modx->resource->get('id');	
$childIDs = $modx->getChildIds($currentID,1,array('context' => 'web'));
$childIDsString = implode(',', $childIDs);
$sql= "SELECT value FROM `modx_site_tmplvar_contentvalues` WHERE contentid IN ($childIDsString) AND tmplvarid = '18'";
mysql_query($sql);

$criteria = $modx->newQuery('modTemplateVarResource');
$criteria->select(array($modx->getSelectColumns('modTemplateVarResource', 'modTemplateVarResource', '', array('id', 'value')))); //Primary key has to be selected
$criteria->where(array('contentid:IN' => $childIDs, //IN accepts array
    'tmplvarid' => 18
));
$results = $modx->getCollection('modTemplateVarResource', $criteria);

return $results;

	foreach($results as $data){
		echo $data."<br>";
	}

foreach($results as $data) 
{
    $out[] = $data->value;
    // OR
    $out[] = $data->get('value');
}
return implode("\n", $out);

$currentID = $modx->resource->get('id');
$childIDs = $modx->getChildIds($currentID,1,array('context' => 'web'));

/* build the query */
$query = $modx->newQuery('modTemplateVarResource', array('tmplvarid' => 18, 'contentid:IN' => $childIDs));
/* select just the value column */
$query->select($modx->getSelectColumns('modTemplateVarResource', '', '', array('value')));
$values = array();
/* prepare and execute a PDOStatement from the $query */
if ($query->prepare() && $query->stmt->execute()) {
    /* use PDOStatement::fetchColumn() to get an array representing 
        the first column of the result set */
    $values = $query->stmt->fetchColumn();
}