diff --git a/ext/mbstring/php_onig_compat.h b/ext/mbstring/php_onig_compat.h
index c97ba0c5cb674..5a1fa8eeaaf1b 100644
--- a/ext/mbstring/php_onig_compat.h
+++ b/ext/mbstring/php_onig_compat.h
@@ -5,4 +5,10 @@
 #define regex_t                     php_mb_regex_t
 #define re_registers                php_mb_re_registers
 
+/* Required for some distros that conditionally override PV_.
+ * As we're in C99 this header is always available. */
+#ifndef HAVE_STDARG_H
+# define HAVE_STDARG_H
+#endif
+
 #endif /* _PHP_ONIG_COMPAT_H */