php - Beginner's question: How to solve the Undefined index problem?

Question

Beginner to PHP and mysql, follow the instructions in the book. But how to solve Undefined index? I checked online but nothing is very clear.
The following is a screenshot of my phpstorm code

< /p>

< /p>

Answer 1

Use isset to determine whether there is this index in $_POST

Answer 2

Do not place the input box inside the td tag

Answer 3

Set error reporting level. Add error_reporting(E_ERROR | E_WARNING | E_PARSE);

to the top of php.ini or php file

Or the variable must be $a=''; this is very troublesome.