I want to use PHP mail() function to send email to hundreds of users, in mail function I can set mail list like:
$sendto="user1@abcd.com,user2@xyz.com,user3@yahoo.com,.....";
But in this way when user receives this email, he can see all the email recipients at the top, I want to hide this info, is it possible to use:
$sendto="To Users<user1@abcd.com,user2@xyz.com,user3@yahoo.com,.....>";

Thanks for any comment.

why not send to some dummy address and then bcc the rest

or why not just loop through it and send it to each user individually, as most spam catchers dont really like BCC's

but otherwise BCC is the only way

this would be the best way. to have a list of the emails in a database and then itterate through the database like while count is less than or equals to the number of emails in the database. email the current count (value stored in the database) something to that effect.

Doing it this way is also more efficient for the server because as your mailing list grows, you can do batch jobs of XXX at a time to lessen server load. It also allows you to customize each email to each user, by using php variables within the mail messages. Makes it seem a lot more personal and you will probably get a better response rate.