Fix failed assertion when promoting Serialize deprecation to exception

Fixes phpGH-15907

Zend/tests/gh15907.phpt

@@ -0,0 +1,18 @@
+--TEST--
+GH-15907: Failed assertion when promoting inheritance error to exception
+--FILE--
+<?php
+
+set_error_handler(function($errno, $msg) {
+    throw new Exception($msg);
+});
+
+class C implements Serializable {
+    public function serialize() {}
+    public function unserialize($serialized) {}
+}
+
+?>
+--EXPECTF--
+Fatal error: During inheritance of C, while implementing Serializable: Uncaught Exception: C implements the Serializable interface, which is deprecated. Implement __serialize() and __unserialize() instead (or in addition, if support for old PHP versions is necessary) in %s:%d
+%a

Zend/zend_interfaces.c

@@ -473,6 +473,10 @@ static int zend_implement_serializable(zend_class_entry *interface, zend_class_e
 	if (!(class_type->ce_flags & ZEND_ACC_EXPLICIT_ABSTRACT_CLASS)
 			&& (!class_type->__serialize || !class_type->__unserialize)) {
 		zend_error(E_DEPRECATED, "%s implements the Serializable interface, which is deprecated. Implement __serialize() and __unserialize() instead (or in addition, if support for old PHP versions is necessary)", ZSTR_VAL(class_type->name));
+		if (EG(exception)) {
+			zend_exception_uncaught_error(
+				"During inheritance of %s, while implementing Serializable", ZSTR_VAL(class_type->name));
+		}
 	}
 	return SUCCESS;
 }