As far as i can tell, there was a bug in the includes/classes/class.phpbb.php file on line 189 the code is:
PHP Code:
$sql "insert into " $this->phpBB['users_table'] . "
                (user_id, username, user_password, user_email, user_regdate)
                values
                ('" 
. (int)$user_id "', '" $nick "', '" md5($password) . "', '" $email_address "', '" time() ."')"
The problem is the "(int)" in the SQL

My fix code is:
PHP Code:
$sql "insert into " $this->phpBB['users_table'] . "
                (user_id, username, user_password, user_email, user_regdate)
                values
                ('" 
$user_id "', '" $nick "', '" md5($password) . "', '" $email_address "', '" time() ."')"
Without this change, the user was not created in my phpBB forums when a user creates their user account in zen-cart. After making this code change it works perfectly for me...

Jadnet