PDO获取输出bool（false）

when i am fetching a database table from PDO fetch then it shows bool(false)

try {
    $stmt = $db->prepare('SELECT * FROM replies WHERE comment_id = :comment_id');
    $stmt->execute(array(':comment_id' => $postcomment['id']));
    $repl = $stmt->fetch(); 
} catch (Exception $e) {
     echo 'Error: '.$e->getMessage();
}
var_dump($repl); ?>

You appear to be expecting an exception when the query fails. That only happens AFAIK when you set the appropriate attribute on the PDO object (either PDOStatement or PDO, see PDO::setAttribute):

$db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

after setting this, all fetch calls (and similar) will throw an exception on error instead of returning some false return value. Then, the try-catch block actually provides some benefits.

Remark: The setAttribute stuff can also be done in the PDO constructor (see PDO::__construct)