You need to follow along with the source code to fully understand this example

The next statements are attempting to echo the value of an undefined constant (abc) and variable ($abc).
You will get the following error messages displayed, because they are undefined.

Notice: Use of undefined constant abc - assumed 'abc' in /home3/msesonli/public_html/HotSauce/Displaying_errors_example.php on line 11
The value of the constant abc is abc

Notice: Undefined variable: abc in /home3/msesonli/public_html/HotSauce/Displaying_errors_example.php on line 12
The value of the variable $abc is

If you turn off the displaying of errors, then you simply do not get the error message.
The value of the abc constant is abc
The value of the $abc variable is

We have now defined the variable $abc with the value "test" and the constant abc with the value "Constant Value Assigned".

The value of the variable $abc is test.
The value of the constant abc is Constant Value Assigned.

You now see the value of the variable and constant.