Problem when exporting MySQL Database to Excel with PHPExcel

I'm trying to export the table from my mysql database using the PHPExcel library, but it does not generate anything for me.

I have this link with which I make the call to the export file:

<a href="exportar.php">Descargar tabla</a>

and I have this file exportar.php:

<?php

error_reporting(E_ALL);
ini_set("display_errors", 1);

 $conexion = mysqli_connect ("localhost", "xxxx", "xxxx");
 mysqli_select_db ("xxxxx", $conexion);

 $sql = "SELECT * FROM solicitudes ORDER BY folio DESC";
 $resultado = mysqli_query ($sql, $conexion) or die (mysqli_error ());
 $registros = mysqli_num_rows ($resultado);

 if ($registros > 0) {
   require_once 'PHPExcel/Classes/PHPExcel.php';
   $objPHPExcel = new PHPExcel();

   //Informacion del excel
   $objPHPExcel->
    getProperties()
        ->setCreator("lahuerta")
        ->setLastModifiedBy("lahuerta")
        ->setTitle("Exportar Base de Datos")
        ->setSubject("Tabla")
        ->setDescription("Documento generado con PHPExcel")
        ->setKeywords("lahuerta  con  phpexcel")
        ->setCategory("solicitudes");    

   $i = 1;    
   while ($registro = mysqli_fetch_object ($resultado)) {

      $objPHPExcel->setActiveSheetIndex(0)
            ->setCellValue('A'.$i, $registro->name);

      $i++;

   }
}
header('Content-Type: application/vnd.ms-excel');
header('Content-Disposition: attachment;filename="tabla.xlsx"');
header('Cache-Control: max-age=0');

$objWriter=PHPExcel_IOFactory::createWriter($objPHPExcel,'Excel2007');
$objWriter->save('php://output');
exit;
mysql_close ();
?>

At the time of execution, he shows me the following warnings:

Warning: mysqli_select_db() expects parameter 1 to be mysqli, string given in /home/exportar.php on line 7

Warning: mysqli_query() expects parameter 1 to be mysqli, string given in /home/exportar.php on line 10

Warning: mysqli_error() expects exactly 1 parameter, 0 given in /home/exportar.php on line 10