singel or double quotes?

Inspired by the new release of ez_sql I also tried ez_results and the Quick Example:
include_once “ez_sql.php”;
include_once “ez_results.php”;
$ezr->query_mysql(“SELECT email, login FROM users”);
$ezr->display();
with a fatal error (blank screen) as result

Mocking around I found that when changing the qoutes to single in:
$ezr->query_mysql('SELECT email, login FROM users');
it worked out fine.

I'm no real programer so I have to ask why?
(and maybe suggest an update in manual for cheaters like me)

/Björn M