include, include_once, require or require_once?

[YANGEM]

I have PHP file where i have defined the server access variables as well as the mysql_connect and mysql_select_db, as this functions are regularly used in almost every page in backend, while i am using include() which is perfectly working for me now, which method or function would you suggest and i would like to know if there is any flaw if i use include() or is it safe to use it?Edit : Keeping in mind ill be using $_SESSIONS variable too.