$ _GET值为空isset if语句

I am creating a messaging system and storing the message on to a database from a form.

<form action = "sendmsg.php" method="POST">
            <input type="text" placeholder="Enter subject" name="subject"><br/><br/>
            <textarea style="height:300px;width:450px;" maxlength="50" placeholder="Enter Message" name="message"></textarea><br/><br/>
            <input type="submit" name="submitmsg" value="Send Message"/>
            <?php
            require("dbcon.php");

            if(isset($_POST['submitmsg'])){
                $to = $_GET['id']; 
                $subject = $_POST['subject'];
                $message = $_POST['message'];
                $sql = "INSERT INTO msg VALUES ('','$to','NOW()','$subject','$message')";
                $send = mysqli_query($con, $sql);
                echo "Done";
                echo $to;
            }

My problem here is once the submit button has been selected, the $_GET['id'] value is empty. The sql query executes but $_GET['id'] is empty. I have tried various ways to get the $_GET['id'] to work like using a double if statement to check $_GET and $_POST but that didn't work anyway.

I am kinda new to php and been doing this on and off basis.

Any suggestions?

You are sending in the data via the form but you are not sending in any URI information for the $_GET array to parse. Also I am assuming that the HTML and PHP are on separate pages if not you will have to do PHP_SELF as your action to post back to itself properly.

You will need to add this to your form action

?id=<?php echo $id ?>

Like so:

<form action = "sendmsg.php?id=<?php echo $id ?>" method="POST">
        <input type="text" placeholder="Enter subject" name="subject"><br/><br/>
        <textarea style="height:300px;width:450px;" maxlength="50" placeholder="Enter Message" name="message"></textarea><br/><br/>
        <input type="submit" name="submitmsg" value="Send Message"/>
        <?php
        require("dbcon.php");

        if(isset($_POST['submitmsg'])){
            $to = $_GET['id']; 
            $subject = $_POST['subject'];
            $message = $_POST['message'];
            $sql = "INSERT INTO msg VALUES ('','$to','NOW()','$subject','$message')";
            $send = mysqli_query($con, $sql);
            echo "Done";
            echo $to;
        }