Is there a way to have a views filter value be determined by PHP? I need to search for fields that are equal to the value of a visitor cookie. How do I get $_COOKIE[somecookie] as the criterion value in the views filter? (I know that I could use views_php to grab all the records and filter out after the fact but that's a lot of records and would be far too slow.)

It would be preferable to be able to do this without writing a custom module. It seems like this would be a common request by admins so I feel I'm missing something obvious.
Setup the page to use a contextual filter per field you want to be searchable. Then for the contextual filter set it to look for the value provided by the URL. Then select PHP Code for WHEN THE FILTER VALUE IS NOT IN THE URL.

In that PHP blob supply code that pulls the field argument from $_COOKIE, if any or return NULL.