我试图按发布日期降序从数据库中获取前x行，但是我发现，如果直接在本地数据库中执行查询，那么查询效果会非常好，而在php中，结果集总是按主键的顺序进行。下面是一段代码：

public static function getList( $numRows=1000000, $order="publicationDate DESC" ) {
$conn = new PDO( db_host, db_user, db_pw );
$sql = "SELECT SQL_CALC_FOUND_ROWS *, UNIX_TIMESTAMP(publicationDate) AS publicationDate FROM blogs
        ORDER BY " . ":order" . " LIMIT :numRows";

$st = $conn->prepare( $sql );
$st->bindValue( ":numRows", $numRows, PDO::PARAM_INT );
$st->bindValue( ":order", $order, PDO::PARAM_STR);
$st->execute();
$list = array();

while ( $row = $st->fetch() ) {
  echo $row['id'];
  $article = new Blogpost( $row );
  $list[] = $article;
}

// Now get the total number of articles that matched the criteria
$sql = "SELECT FOUND_ROWS() AS totalRows";
$totalRows = $conn->query( $sql )->fetch();
$conn = null;
return ( array ( "results" => $list, "totalRows" => $totalRows[0] ) );
}

运行时，回声的输出如下：50 51 52 53 54
数据库是使用此查询创建的（以显示结构）：

DROP TABLE IF EXISTS blogs;
 CREATE TABLE blogs
(
  id              smallint unsigned NOT NULL auto_increment,
  publicationDate date NOT NULL,
  title           varchar(255) NOT NULL,
  summary         text NOT NULL,
  content         mediumtext NOT NULL,
  views           int(11) NOT NULL,
  previewPic      varchar(255) NOT NULL,

  PRIMARY KEY     (id)
);

我试过运行不同的查询，它总是按主键的升序返回。为什么？