Can we pass an array as parameter in any function in PHP?

Yes, you can do that.

function sendemail($id_list,$userid){
    foreach($id_list as $id) {
        printf("$id\n"); // Will run twice, once outputting id1, then id2
    }
}

$idl = Array("id1", "id2");
$uid = "userID";
sendemail($idl, $uid);

Yes, you can safely pass an array as a parameter.

You can pass an array as an argument. It is copied by value (or COW'd, which essentially means the same to you), so you can array_pop() (and similar) all you like on it and won't affect anything outside.

function sendemail($id, $userid){
    // ...
}

sendemail(array('a', 'b', 'c'), 10);

You can in fact only accept an array there by placing its type in the function's argument signature...

function sendemail(array $id, $userid){
    // ...
}

You can also call the function with its arguments as an array...

call_user_func_array('sendemail', array('argument1', 'argument2'));

even more cool, you can pass a variable count of parameters to a function like this:

function sendmail(...$users){
   foreach($users as $user){

   }
}

sendmail('user1','user2','user3');