how to preserver old option while posting to options.php

Hi,

im new to this and i got a problem with an plugin admin option page post
to options.php to save new settings to database

the problem is that options.php overwrite all the content of the db label

so i instead i post to a custom php file to get old option compare to new one’s and make new $_POST array with everything have to be in the post it back to options.php

but my prob is when i post to my new custom file… i can’t use any of wordpress object methods such as get_option() so what im doing wrong or there is probably an other way of doing this

thanks