Problems related to parameter control transferred from a. php to B. php

The control problem of passing parameters from a. php to B. php ~ A. php contains a form pointing to action = & quot; B. php & quot; and then submitted through post. B. php receives the data in these forms (only two data types are string types), and then transmits parameter control issues to the database from a. php to B. php ~
A. php has a form that points to action = "B. php" and then submits the form through post. B. php receives the data in these forms (only two data types are string types), then executes the INSERT operation to the database, and then uses the header ("Location:. php ") returns. php page.
The problem is:
How can I ensure that B. php can only be accessed through a. php form submission, and prohibit all other access methods?
For example, check whether the form variable exists?

------ Solution --------------------
Determine $ _ SERVER ['http _ referer '].