i got this error:1045error_access denied for "myuser"@myhostaddress (using password:Yes)

Question

im trying to connect and check user information in c# via php and when i use webrequest i got this error:

i got this error:1045error_access denied for "myuser"@myhostaddress (using password:Yes)

its my c# related code

MyWebRequest myRequerst = new MyWebRequest("http://mysite", "POST", "apikey=compo#1&DBKEY=loginrequest&trans_loguser=" + usernametxt.Text + "&trans_logpass=" + passtxt.Text);

and its my php connection string

$connection_obj = mysqli_connect("my site", "myusername", "mypassword", "mydbname");
        if (!$connection_obj) 
        {
            echo "Error No: " . mysqli_connect_errno();
            echo "Error Description: " . mysqli_connect_error();

            exit;
        }   

what should i do???