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

Question

I am new to PHP and mysql. I followed the instructions in the book. But how to solve Undefined index? I checked online but nothing is very clear. Below is a screenshot of my phpstorm code

仅有的幸福 · Answer

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

PHP中文网 · Answer

Do not place the input box inside the td tag

世界只因有你 · Answer

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.