diff --git a/CODING_STANDARDS.md b/CODING_STANDARDS.md
index 25ecd75cb07bc..61e878fe855b2 100644
--- a/CODING_STANDARDS.md
+++ b/CODING_STANDARDS.md
@@ -9,6 +9,10 @@ rewritten to comply with these rules.
 
 1. Document your code in source files and the manual. (tm)
 
+1. PHP is implemented in C99.  The optional fixed-width integers from
+    stdint.h (int8_t, int16_t, int32_t, int64_t and their unsigned
+    counterparts) must be available.
+
 1. Functions that are given pointers to resources should not free them.
 
     For instance, `function int mail(char *to, char *from)` should NOT free `to`