如何通过Username和Key选择MySql表中的行并在该行中插入数据

I want to make a PHP Script for selecting a row by Username and Key from the Table, then inserting data into that row.

How would I go about this?

As far as I understand, you're trying to update a specific row. That can be done with a simple SQL statement.

UPDATE table_name SET user_ip = input_user_i WHERE username = your_username AND key = your_key

I think / hope you know how to query the database with PHP.