PHP MySQLi没有更新

I have been trying to make a form where I can update two fields one field is going be admin_welcomebox and admin_author and I'm trying update it by the id so here go my code

<div class="col-lg-6">                      
    <div class="panel panel-color panel-inverse">                               
        <div class="panel-heading">
            <h3 class="panel-title">Welcome Box Update</h3>
        </div>

        <?php
            if(isset($_POST["submit"])){
                $servername = "localhost";
                $username = "trres";
                $password = "sss";
                $dbname = "txxxs";

                // Create connection
                $conn = new mysqli($servername, $username, $password, $dbname);
                // Check connection
                if ($conn->connect_error) {
                    die("Connection failed: " . $conn->connect_error);
                }

                $sql = "UPDATE admin_news SET welcomebox = '{$admin_news}' SET author = {$admin_author} id='{$id}'";

                if ($conn->query($sql) === TRUE) {
                    echo "<h4 class='bg-success'>You have updated admin welcome box.</h4>";
                } else {
                    echo "<script type= 'text/javascript'>alert('Error: " . $sql . "<br>" . $conn->error."');</script>";
                }
                $conn->close();
            }
        ?>
        <div class="panel-body">
            <form method="post" action="">
                <div class="form-group">
                    <label for="welcomebox">Welcome Box</label>
                    <textarea type="text" name="welcomebox" id="welcomebox" placeholder="Enter Your Message" class="form-control"></textarea>
                </div>      
                <div class="form-group">
                    <label for="author">Author Name</label>
                    <input type="text" name="author" id="author" placeholder="Author Name" class="form-control" / >
                </div>                                          
                <div class="form-group text-right m-b-0">
                    <button class="btn btn-primary waves-effect waves-light" type="submit" name="submit" id="submit">
                        Update Info
                    </button>
                </div>
            </form>                                 
        </div>                                              
    </div>                              
</div>

When I try update it just refresh the page nothing else.

You didn't define $admin_news ,$admin_author and $id as well. define first.

Try this code :-

<div class="col-lg-6">                      
<div class="panel panel-color panel-inverse">                               
    <div class="panel-heading">
        <h3 class="panel-title">Welcome Box Update</h3>
    </div>

    <?php
        if(isset($_POST["submit"])){
            $servername = "localhost";
            $username = "trres";
            $password = "sss";
            $dbname = "txxxs";

            // Create connection
            $conn = new mysqli($servername, $username, $password, $dbname);
            // Check connection
            if ($conn->connect_error) {
                die("Connection failed: " . $conn->connect_error);
            }
            $id=$_POST['id'];
            $admin_news=$_POST['welcomebox']; 
            $admin_author=$_POST['author']; 

            $sql = "UPDATE admin_news SET welcomebox = '$admin_news', author = $admin_author where id=$id";

            if ($conn->query($sql) === TRUE) {
                echo "<h4 class='bg-success'>You have updated admin welcome box.</h4>";
            } else {
                echo "<script type= 'text/javascript'>alert('Error: " . $sql . "<br>" . $conn->error."');</script>";
            }
            $conn->close();
        }
    ?>


    <div class="panel-body">
            <form method="post" action="">
  <input type="hidden" name="id" value="<?php echo $id; ?>" /> <!-- put here your id  --> 
               <div class="form-group">
                    <label for="welcomebox">Welcome Box</label>
                    <textarea type="text" name="welcomebox" id="welcomebox"                       placeholder="Enter Your Message" class="form-control"></textarea>
                </div>      
                <div class="form-group">
                    <label for="author">Author Name</label>
                    <input type="text" name="author" id="author" placeholder="Author Name" class="form-control" / >
                </div>                                          
                <div class="form-group text-right m-b-0">
                    <button class="btn btn-primary waves-effect waves-light" type="submit" name="submit" id="submit">
                        Update Info
                    </button>
                </div>
            </form>                                 
        </div>                                              
    </div>                              
</div>

Your query has invalid syntax. This is wrong:

UPDATE admin_news SET welcomebox = '{$admin_news}' SET author = {$admin_author} id='{$id}'

The right MySQL syntax for UPDATE is

UPDATE admin_news SET welcomebox = 'value', author = 'value' WHERE id='id'