diff --git a/CODING_STANDARDS.md b/CODING_STANDARDS.md
index 5fb2586eadd78..c599194ed50e3 100644
--- a/CODING_STANDARDS.md
+++ b/CODING_STANDARDS.md
@@ -9,12 +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.
+1. PHP is implemented in C11.
     For instance, the optional fixed-width integers from
     stdint.h (int8_t, int16_t, int32_t, int64_t and their unsigned
     counterparts) are supposed to be available.
-    However, some features (most notably variable-length arrays) which are not
-    supported by all relevant compilers, must not be used.
 
 1. Functions that are given pointers to resources should not free them.
 
diff --git a/Zend/Zend.m4 b/Zend/Zend.m4
index b7b44fb140872..7ffdfcbf9b583 100644
--- a/Zend/Zend.m4
+++ b/Zend/Zend.m4
@@ -1,5 +1,28 @@
 dnl This file contains Zend specific autoconf macros.
 
+dnl
+dnl ZEND_CHECK_TYPEDEF_REDECL
+dnl
+dnl Check whether the compiler supports typedef redeclaractions. For example,
+dnl GCC 4.2 on Mac OS X 10.6 emits a redefinition of typedef error by default.
+dnl
+AC_DEFUN([ZEND_CHECK_TYPEDEF_REDECL], [dnl
+AC_CACHE_CHECK([if compiler allows typedef redeclarations],
+  [php_cv_have_typedef_redecl],
+  [AC_LINK_IFELSE([AC_LANG_PROGRAM([], [dnl
+    typedef int mytype;
+    typedef int mytype;
+    mytype var;
+    (void)var;
+  ])],
+  [php_cv_have_typedef_redecl=yes],
+  [php_cv_have_typedef_redecl=no])])
+AS_VAR_IF([php_cv_have_typedef_redecl], [no],
+  [AC_MSG_FAILURE(m4_text_wrap([
+    Compiler does not support typedef redeclarations. Does it support C11?
+  ]))])
+])
+
 dnl
 dnl ZEND_CHECK_FLOAT_PRECISION
 dnl
@@ -157,6 +180,7 @@ AC_CHECK_FUNC([sigsetjmp],,
     [AC_MSG_FAILURE([Required sigsetjmp not found.])],
     [#include <setjmp.h>])])
 
+ZEND_CHECK_TYPEDEF_REDECL
 ZEND_CHECK_STACK_DIRECTION
 ZEND_CHECK_FLOAT_PRECISION
 ZEND_DLSYM_CHECK
@@ -206,6 +230,10 @@ AX_CHECK_COMPILE_FLAG([-Wstrict-prototypes],
 AX_CHECK_COMPILE_FLAG([-fno-common],
   [CFLAGS="-fno-common $CFLAGS"],,
   [-Werror])
+dnl Suppress the Clang typedef redefinition warnings if it has enabled C99 mode.
+AX_CHECK_COMPILE_FLAG([-Wno-typedef-redefinition],
+  [CFLAGS="$CFLAGS -Wno-typedef-redefinition"],,
+  [-Werror])
 
 ZEND_CHECK_ALIGNMENT
 ZEND_CHECK_SIGNALS