如何告诉PHP在评论中获取并发布用户名？

Question

我是PHP的新手，但是我需要帮助，我想在发表评论的过程中从其数据库中获取用户名和图像，并将其插入要发表评论的登录用户中。

这是我的代码

<?php
include('dbconnect.php');
$error = '';
$username = '';
$post = '';

$username = $_POST["username"];
$user_image = $_POST["user_image"];
$post = $_POST["post"];
$time = $_POST["time"];

if(empty($_POST["post"]))
{
$error .= '<p class="text-danger">Empty post!</p>';
}
else 
{
$post = $_POST["post"];
}

if($error == '')
{
$query = "
INSERT INTO comments
(comment_id, username, user_image, post, time)
VALUES (:comment_id, :username, :user_image, :post, :time)";
$statement = $con->prepare($query);
$statement->execute(
    array(
    ':comment_id' => '0',
    ':post' => $post,
    ':username' => $username
    ':user_image' => $user_image,
    ':post' => $post,
    'time' = $time
    )

    );
    $error = '<label class="text-success">Comment Added</label>';
}

$data= array(
'error' => $error
);

echo json_encode($data);
?>