php code

Here is a code snippet from my project
$db = mysql_pconnect('', 'name', 'pass');
$sd=  mysql_select_db("books",$db);

the first part works perfectly and it connects to the mysql
however it will not choose the database i need.

it's blank if i've used it right
  $sd=  mysql_select_db("nuclear_books",$db);
  echo mysql_error();
  if ($sd = "false")
  {
    echo "selecting database failed <br>";
  }
It displays selecting database failed but mysql_error isn't returning anything