php - Parse error: syntax error, unexpected '[' with mysql_fetch_assoc I think

I have a client who's hosting was changed to a new hosting
server. In the past they had someone build them a small database project which worked
before, but is no longer working in the new hosting environment. Naturally, I didn't
have anything to do with the scripts and the original coder is telling them he it too
busy to look at it.

The section failing is this
line of code.

$registerquery =
            mysql_query("INSERT INTO users (".implode(array_keys($vals), ', ').")
            VALUES('".implode(array_values($vals), "', '")."')");
 if ($registerquery)
            {

 $new_user_id = mysql_fetch_assoc(mysql_query("SELECT id FROM
            users WHERE email = '$email'"))['id'];
 log_in($new_user_id);

            ...

With the specific
line the error is pointing to
being:

$new_user_id =
            mysql_fetch_assoc(mysql_query("SELECT id FROM users WHERE email =
            '$email'"))['id'];

The
error is

Parse error: syntax
            error, unexpected '[' in ...
            

pointing to the
specific line with the mysql_fetch_assoc in it which is what is having the syntax error.

Someone have a suggestion on what I could do to
fix it.