Backported from 5.6.27 by Remi.


From 33a8af0510c5899cbf9148f53da08cf4f2df0013 Mon Sep 17 00:00:00 2001
From: Stanislav Malyshev <stas@php.net>
Date: Tue, 20 Sep 2016 22:59:12 -0700
Subject: [PATCH] Fix bug #73073 - CachingIterator null dereference when
 convert to string

---
 ext/spl/spl_iterators.c     | 254 +++++++++++++++++++++++---------------------
 ext/spl/tests/bug73073.phpt |   9 ++
 2 files changed, 141 insertions(+), 122 deletions(-)
 create mode 100644 ext/spl/tests/bug73073.phpt

diff --git a/ext/spl/spl_iterators.c b/ext/spl/spl_iterators.c
index a023b11..c6d03e0 100644
--- a/ext/spl/spl_iterators.c
+++ b/ext/spl/spl_iterators.c
@@ -2784,15 +2784,25 @@ SPL_METHOD(CachingIterator, __toString)
 
 	SPL_FETCH_AND_CHECK_DUAL_IT(intern, getThis());
 
+	if (!spl_caching_it_valid(intern TSRMLS_CC)) {
+		RETURN_EMPTY_STRING();
+	}
+
 	if (!(intern->u.caching.flags & (CIT_CALL_TOSTRING|CIT_TOSTRING_USE_KEY|CIT_TOSTRING_USE_CURRENT|CIT_TOSTRING_USE_INNER)))	{
 		zend_throw_exception_ex(spl_ce_BadMethodCallException, 0 TSRMLS_CC, "%s does not fetch string value (see CachingIterator::__construct)", Z_OBJCE_P(getThis())->name);
 		return;
 	}
 	if (intern->u.caching.flags & CIT_TOSTRING_USE_KEY) {
+		if (!intern->current.key) {
+			RETURN_EMPTY_STRING();
+		}
 		MAKE_COPY_ZVAL(&intern->current.key, return_value);
 		convert_to_string(return_value);
 		return;
 	} else if (intern->u.caching.flags & CIT_TOSTRING_USE_CURRENT) {
+		if (!intern->current.data) {
+			RETURN_EMPTY_STRING();
+		}
 		MAKE_COPY_ZVAL(&intern->current.data, return_value);
 		convert_to_string(return_value);
 		return;
@@ -2800,7 +2810,7 @@ SPL_METHOD(CachingIterator, __toString)
 	if (intern->u.caching.zstr) {
 		RETURN_STRINGL(Z_STRVAL_P(intern->u.caching.zstr), Z_STRLEN_P(intern->u.caching.zstr), 1);
 	} else {
-		RETURN_NULL();
+		RETURN_EMPTY_STRING();
 	}
 } /* }}} */
 
diff --git a/ext/spl/tests/bug73073.phpt b/ext/spl/tests/bug73073.phpt
new file mode 100644
index 0000000..218a28e
--- /dev/null
+++ b/ext/spl/tests/bug73073.phpt
@@ -0,0 +1,9 @@
+--TEST--
+Bug #73073: CachingIterator null dereference when convert to string
+--FILE--
+<?php
+$it = new CachingIterator(new ArrayIterator(array()), CachingIterator::TOSTRING_USE_KEY);
+var_dump((string)$it);
+?>
+--EXPECT--
+string(0) ""
-- 
2.1.4