Error creating mysqli connection

I'm trying to create a connection through MYSQLI plus this one giving error Fatal error: Call to undefined function mysqli_connect () in / home What can this be?

$link = mysqli_connect("localhost", "xx", "xxx", "xxx");
if (!$link) {
  echo "Error: Unable to connect to MySQL." . PHP_EOL;
  echo "Debugging errno: " . mysqli_connect_errno() . PHP_EOL;
  echo "Debugging error: " . mysqli_connect_error() . PHP_EOL;
  exit;
}

echo "Success: A proper connection to MySQL was made! The my_db database is great." . PHP_EOL;
echo "Host information: " . mysqli_get_host_info($link) . PHP_EOL;

mysqli_close($link);