How to make a database backup with PHP?

Question

PHP

#code.. (PHP)

<?php 
$backup_folder = '/www/site.ru/backup';    // куда сохранятся файлы - полный путь ИЛИ  /backup
$backup_name = 'datebase_backup_' . date("Y-m-d");    // имя нового файла

$db_host = 'localhost';
$db_user = 'petuh';
$db_password = '12345';
$db_name = 'petuh';

function backupDB($backup_folder, $backup_name){
  global $db_host;
  global $db_user;
  global $db_password;
  global $db_name;
    $fullFileName = $backup_folder . '/' . $backup_name . '.sql';
    $command = 'mysqldump -h' . $db_host . ' -u' . $db_user . ' -p' . $db_password . ' ' . $db_name . ' > ' . $fullFileName;
    shell_exec($command);
    return $fullFileName;

}
backupDB($backup_folder, $backup_name);

The script does not do its job.
Things are done on Reg.ru