fix: make generate abstract classes actually abstract

Signed-off-by: Dalibor Karlović <[email protected]>

Author: dkarlovi

tool/resources/org/antlr/v4/tool/templates/codegen/PHP/PHP.stg

@@ -97,7 +97,7 @@ use Antlr\\Antlr4\\Runtime\\Tree\\TerminalNode;
  * which can be extended to create a listener which only needs to handle a subset
  * of the available methods.
  */
-class <file.grammarName>BaseListener implements <file.grammarName>Listener
+abstract class <file.grammarName>BaseListener implements <file.grammarName>Listener
 {
 	<file.listenerNames:{lname |
 /**
@@ -188,7 +188,7 @@ use Antlr\\Antlr4\\Runtime\\Tree\\AbstractParseTreeVisitor;
  * which can be extended to create a visitor which only needs to handle a subset
  * of the available methods.
  */
-class <file.grammarName>BaseVisitor extends AbstractParseTreeVisitor implements <file.grammarName>Visitor
+abstract class <file.grammarName>BaseVisitor extends AbstractParseTreeVisitor implements <file.grammarName>Visitor
 {
 	<file.visitorNames:{lname |
 /**