Is there a good checklist for creating a secure form that talks to a CRM [on hold]

I am creating a form with over 70 fields. I have created my functions in PHP for validation. I have created a MySQL database to receive the data and I will be integrating an api to output the data to a CRM. Is there anything I'm leaving out of this process?

Also, a mentor recommended that I use a PHP framework. I don't understand why that would be necessary.

Thanks for your help!