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SELECT * FROM wp_users where user_nicename = '".$username."'"

<?php
//variables y conexion mysql
$hostname="localhost";
$username="myinvestpa";
$password="jPd4qB06";
$dbname="investpainbbdd";

$conn = mysqli_connect($hostname,$username,$password) or die ("<html><script     language='JavaScript'>alert('Unable to connect to database! Please try again later.'),history.go(-1)</script></html>");
mysqli_select_db($conn,$dbname);

//Realizamos la consulta sql

$query = "SELECT * FROM wp_users where user_nicename = '".$username."'";
$results = mysqli_query($conn,$query) or die('ok');


  if(mysqli_num_rows(@$results) != 0){
    echo '<div id="Error">ID de patrocinador Correcto</div>';
  }
 if(mysqli_num_rows(@$results) == 0){
    echo '<div id="Succes">ID de patrocinador Incorrecto</div>';
 }
 ?>