为什么mysqli num_rows总是返回0？

您需要先调用

MySqli_Stmt::store_result()

if($stmt = $mysqli->prepare("SELECt id, title, visible, parent_id FROM content WHERe parent_id = ? ORDER BY page_order ASC;")){      $stmt->bind_param('s', $data->id);      $stmt->execute();    $stmt->store_result(); <-- This needs to be called here!    $num_of_rows = $stmt->num_rows;      $stmt->bind_result($child_id, $child_title, $child_visible, $child_parent);    while($stmt->fetch()){        //pre    }    echo($num_of_rows);    $stmt->close();}

请参阅文档

MySQLi_Stmt->num_rows