Add final keyword to classes

exceptions/MDConsoleInvalidHeadingNumber.php

@@ -4,7 +4,7 @@
 /**
  * Custom exception class for invalid page parameters.
  */
-class MDConsoleInvalidHeadingNumber extends Exception {
+final class MDConsoleInvalidHeadingNumber extends Exception {
 
     /**
      * Error message.

src/MDConsole.php

@@ -9,7 +9,7 @@
 /**
  * Interface for CLI outputs of museum-digital's dev and admin tools.
  */
-class MDConsole {
+final class MDConsole {
 
     const STATUS_NOTICE = 0;
     const STATUS_UPDATE = 1;

src/MDOutputHandler.php

@@ -9,7 +9,7 @@
 /**
  * Class for handling outputs and logs for museum-digital's background tools.
  */
-class MDOutputHandler {
+final class MDOutputHandler {
 
     /*
      * Variables