diff --git a/scripts/phpize.m4 b/scripts/phpize.m4
index c6a57302a43..6773a56734e 100644
--- a/scripts/phpize.m4
+++ b/scripts/phpize.m4
@@ -92,6 +92,7 @@ PHP_SUBST(CFLAGS_CLEAN)
 PHP_SUBST(CPP)
 PHP_SUBST(CPPFLAGS)
 PHP_SUBST(CXX)
+PHP_SUBST(CXXFLAGS)
 PHP_SUBST(DEFS)
 PHP_SUBST(EXTENSION_DIR)
 PHP_SUBST(EXTRA_LDFLAGS)