Added tests demonstrating the same effect with abstracts

Zend/tests/bug73987.phpt

@@ -1,5 +1,5 @@
 --TEST--
-Bug #73987 (Method compatibility check looks to original definition and not parent)
+Bug #73987 (Method compatibility check looks to original definition and not parent - nullability interface)
 --FILE--
 <?php
 

Zend/tests/bug73987_1.phpt

@@ -1,5 +1,5 @@
 --TEST--
-Bug #73987 (Method compatibility check looks to original definition and not parent)
+Bug #73987 (Method compatibility check looks to original definition and not parent - return types interface)
 --FILE--
 <?php
 

Zend/tests/bug73987_2.phpt

@@ -0,0 +1,20 @@
+--TEST--
+Bug #73987 (Method compatibility check looks to original definition and not parent - nullabilty abstract)
+--FILE--
+<?php
+
+abstract class A {
+    abstract function example($a, $b, $c);
+}
+
+class B extends A {
+    function example($a, $b = null, $c = null) { }
+}
+
+class C extends B {
+    function example($a, $b, $c = null) { }
+}
+
+?>
+--EXPECTF--
+Fatal error: Declaration of C::example($a, $b, $c = NULL) must be compatible with B::example($a, $b = NULL, $c = NULL) in %s

Zend/tests/bug73987_3.phpt

@@ -0,0 +1,20 @@
+--TEST--
+Bug #73987 (Method compatibility check looks to original definition and not parent - return types abstract)
+--FILE--
+<?php
+
+abstract class A {
+    abstract function example();
+}
+
+class B extends A {
+    function example(): int  { }
+}
+
+class C extends B {
+    function example(): string { }
+}
+
+?>
+--EXPECTF--
+Fatal error: Declaration of C::example(): string must be compatible with B::example(): int in %s