Match one field with Another on Submission

Hi,

I have 2 fields; one text, one drop down.
I have conditions placed where either one or the other field must be filled.

When the form is submitted, if the text field is empty, I want the drop down value to match the text, because this text field is going to be populated into a CRM, otherwise the text field is already populated and it will stay the same.

I believe my request is best solved with a php filter, but I am not familiar with php.

Thanks.