Upload and save a file in to a database  using PHP and MySQL. let's implement upload a file.

create an upload form input file type

<form method="post" action="" enctype="multipart/form-data">
<input type="file" name="File" id="File">
<button type="submit" class="btn" name="btn-upload">Upload</button>
</form>

I have not applied on upload file type extension but you can add a restriction based on your need

if(isset($_POST['btn-upload']))
{    
 global $conn, $errors, $file;  
 $file = rand(1000,100000)."-".$_FILES["File"]['name'];
 $file_loc = $_FILES["File"]['tmp_name'];
 $file_size = $_FILES["File"]['size'];
 $file_type = $_FILES["File"]['type'];
 $folder="uploads/";
 // new file size in KB
 $new_size = $file_size/1024;  
 // new file size in KB
 // make file name in lower case
 $new_file_name = strtolower($file);
 // make file name in lower case
 $final_file=str_replace(' ','-',$new_file_name);
 if (empty($file)) { array_push($errors, "File is required"); }
  if(!move_uploaded_file($file_loc,$folder.$final_file))
   {
    array_push($errors, "Failed to upload file. Please check file settings for your server");
	}
  // Ensure that no post is saved twice. 
		$post_check_query = "SELECT * FROM uploadfile WHERE File='$final_file' LIMIT 1";
		$result = mysqli_query($conn, $post_check_query);
        if (mysqli_num_rows($result) > 0) { // if post exists
			array_push($errors, "A file already exists with that file name.");
		}
	// create post if there are no errors in the form
   if (count($errors) == 0) {
  $sql="INSERT INTO uploadfile(File,type,size,downloads,Date) VALUES('$final_file','$file_type','$new_size',0,NOW())";
  mysqli_query($conn, $sql);
  $_SESSION['upload_message'] = "File Uploaded successfully";
	header('location: upload.php');
	exit(0);
	}
}

then download the file when a user click a download link

// Downloads files
if (isset($_GET['file_id'])) {
    $id = $_GET['file_id'];
    // fetch file to download from database
    global $conn;
    $sql = "SELECT * FROM uploadfile WHERE FID=$id";
    $result = mysqli_query($conn, $sql);
    $file = mysqli_fetch_assoc($result);
    $filepath = 'admin/uploads/' . $file['File'];
if (file_exists($filepath)) {
        header('Content-Description: File Transfer');
        header('Content-Type: application/octet-stream');
        header('Cache-Control: must-revalidate');
        header('Expires: 0');
        header('Content-Disposition: attachment; filename="'.basename($filepath).'"');
        header('Content-Length: ' . filesize('admin/uploads/' . $file['File']));
        header('Pragma: public');
        flush();
        readfile('admin/uploads/' . $file['File']);
        // Now update downloads count
        $newCount = $file['downloads'] + 1;
        $updateQuery = "UPDATE uploadfile SET downloads=$newCount WHERE FID=$id";
        mysqli_query($conn, $updateQuery);
        exit(0);
    }
    else{
    $_SESSION['message'] = "File does not exist.";
    header('location: ' . BASE_URL . '#.php');
	exit(0);
    }
}