This has been fixed in PHP 7.4, add a test for it.
--- /dev/null
+--TEST--
+Bug #63816: implementation child interface and after parent cause fatal error
+--FILE--
+<?php
+
+interface RootInterface
+{
+ function foo();
+}
+
+interface FirstChildInterface extends RootInterface
+{
+ function foo();
+}
+
+interface SecondChildInterface extends RootInterface
+{
+ function foo();
+}
+
+class A implements FirstChildInterface, SecondChildInterface
+{
+ function foo()
+ {
+ }
+}
+
+class B implements RootInterface, FirstChildInterface
+{
+ function foo()
+ {
+ }
+}
+
+class C implements FirstChildInterface, RootInterface
+{
+ function foo()
+ {
+ }
+}
+
+?>
+===DONE===
+--EXPECT--
+===DONE===