每当我更改用户名时，我的系统就会出现问题。在我的数据库中，用户名是主键。请参见下面的屏幕截图。这就是它所做的。

如果我退出登录，一切都会显示出来。似乎每次我更改用户名时都无法连接到数据库。
这是我更新配置文件的代码：

<?php

session_start();

require 'config.php';

$user=$_SESSION['lec']; 

$update = $conn->real_escape_string($_POST['update']);
$phone = $conn->real_escape_string($_POST['phone']);
$email = $conn->real_escape_string($_POST['email']);
$pswd = $conn->real_escape_string($_POST['pswd']);
$pswd2 = $conn->real_escape_string($_POST['pswd2']);
$name = $conn->real_escape_string($_POST['name']);
$username = $conn->real_escape_string($_POST['username']);
$dept = $conn->real_escape_string($_POST['dept']);

if ($pswd!=$pswd2)
{
	$_SESSION["message"] = "Passwords do not match. Please try again.";
	header("location:lecprofile?error");
}

else
{
	$pswd = password_hash($pswd, PASSWORD_BCRYPT);

	$sql1= "UPDATE lecturer SET email='$email' WHERE username='$user'";
	$sql2 = "UPDATE lecturer SET phonenumb='$phone' WHERE username='$user'";
	$sql4 = "UPDATE lecturer SET username='$username' WHERE username='$user'";
	$sql5 = "UPDATE lecturer SET name='$name' WHERE username='$user'";
	$sql6 = "UPDATE lecturer SET dept='$dept' WHERE username='$user'"; 
	$result = $conn->query($sql1) && $conn->query($sql2) && $conn->query($sql4) && $conn->query($sql5) && $conn->query($sql6);

if (empty($pswd) || empty($pswd2))
{
	header("location:lecprofile");
}
else
{
		$sql3= "UPDATE lecturer SET pswd='$pswd' WHERE username='$user'";

}
	$passresult=$conn->query($sql3);
		if($result==TRUE || $passresult== TRUE)
		{
		$_SESSION['message'] = "Profile Updated!";
		header("location:lecprofile?done");
		}
}

if(isset($_POST['submit'])){
 //Targeting Folder
 $target="propics/";
 $target=$target.basename($_FILES['propic']['name']);
 //Getting Selected image Type
 $type=pathinfo($target,PATHINFO_EXTENSION);
 //Allow Certain File Format To Upload
 if($type!='jpg' && $type!='jpeg' && $type!='JPG' && $type!='PNG' && $type!='GIF'  && $type!='png' && $type!='gif'){
  echo "Only JPG,JPEG,PNG and GIF file format are allowed to Upload";
 }
 else{
 	//lmit file size
 	if ($_FILES['propic']['size'] > 5000000) {
        echo "Sorry, your image is too large.";        
        }

 else{
  //checking for Exsisting image Files
  if(file_exists($target)){
   echo "File Already Exist";
   }else{

   //Moving The image file to Desired Directory
  $upload_success=move_uploaded_file($_FILES['propic']['tmp_name'],$target);
  if($upload_success==TRUE){
   //Getting Selected image Information

    //renaming the file to the username to avoid conflict when uploading
    $name=$user.".".$type;
    rename("propics/".$_FILES['propic']['name'],"propics/".$name);	
    //					 
    $sql= "UPDATE lecturer SET propic='$name' WHERE username='$user'";
   $result = $conn->query($sql);
   if($result==TRUE){
    clearstatcache();
	$_SESSION['message']= "Profile picture updated successfully!";
   header("location:lecprofile.php?done");
   }
  }
  }
  }
 }
}

?>

用于在数据库中显示数据的代码。

<?php

   require 'config.php';

    if ($_SESSION['logged_in'] != 1 ){
        $_SESSION['message'] = "Please login!"; 
         header("location: /prac?error");
       }
      else
      { 
        $user=$_SESSION['lec'];
      }
     if (isset($_GET['done']))
     {
     	include 'done.php';
     }
     if (isset($_GET['error']))
     {
     	include 'error.php';
     }
    ?>

	<table >
		<tr>
			<td width="20%" align="center">
				<?php $sql = "SELECT propic FROM lecturer WHERE username='$user'";
					  $result = $conn->query($sql);
					  $row = $result->fetch_assoc();
				?>
				<img id="mainpropic" src="propics/<?php echo $row['propic'];?>" width="200px" height="200px" >
			  </td>
				<td >&nbsp </td>

			<td>
			<form action="" method="post">
				<table cellpadding="2" >
					<tr>
						<td>

							User Name:
						</td>
						<td>
							<?php echo $_SESSION['lec'];?>
						</td>
					</tr>
					<tr>
						<td>
							EC Number: 
						</td>
						<td>
							<?php 
							$sql = "SELECT ecnumber FROM lecturer WHERE username='$user'";
							$result = $conn->query($sql);
							$row = $result->fetch_assoc();
							 echo $row["ecnumber"];?>
						</td>
					</tr> 
					<tr>
						<td>
							Name:
						</td>
						<td>
							<?php 
							$sql = "SELECT name FROM lecturer WHERE username='$user'";
							$result = $conn->query($sql);
							$row = $result->fetch_assoc();
							 echo $row["name"];
							 ?>
						</td>
					</tr>
					<tr>
						<td>
							Email:
						</td>
						<td>
							<?php 
							$sql = "SELECT email FROM lecturer WHERE username='$user'";
							$result = $conn->query($sql);
							$row = $result->fetch_assoc();
							 echo $row["email"];?>
						</td>
					</tr>
					<tr>
						<td>
							Phone Number:
						</td>
						<td>
							<?php 
							$sql = "SELECT phonenumb FROM lecturer WHERE username='$user'";
							$result = $conn->query($sql);
							$row = $result->fetch_assoc();
							 echo $row["phonenumb"];?>
						</td>
					</tr>
					<tr>
						<td>
							Department:
						</td>
						<td>
							<?php 
							$sql = "SELECT dept FROM lecturer WHERE username='$user'";
							$result = $conn->query($sql);
							$row = $result->fetch_assoc();
							 echo $row["dept"];?>
						</td>
					</tr>

					<tr>

					</tr>
					<tr>
						<td>

							<input type="button" value="Edit Profile" class="btn btn-primary btn-sm" data-toggle="modal" data-target="#Modal">
						</td>
						<td>
						</td>
					</tr>

				</table>