connected to the database BUT it shows [Access denied for user 'www-data'@'localhost' (using password: NO)]

Question

I am using PhpMyAdmin in Ubuntu. trying to access my DB by php, I find something wrong.

here is my code:

<?php
$con=mysqli_connect(HOST,USER,PASSWORD);
    if(!$con){
            echo "sign in failure </br>";
        }else{
            echo "sign in success </br>";
        }
$dbconnect=mysqli_select_db($con, DB) or die ('Select Error:'.mysql_error());
    if(!$dbconnect){
            echo "database disconnect </br>";
        }else{
            echo "database connected </br>";
        }
$sql_query="SELECT * FROM `goodstart`"; 
    if(!$sql_query){
            echo "Table disconnect </br>";
        }else{
            echo "Table connected </br>";
        }
$result=mysql_query($sql_query) or die ('Select Error:'.mysql_error());
while($row=mysql_fetch_assoc($result)){
    $output[]=$row;
}
    echo $output;
mysql_close($con);
?>

when i run it on my browser, it shows:

sign in success

database connected

Table connected

Select Error:Access denied for user 'www-data'@'localhost' (using password: NO) how can i solve the problem?