diff --git a/configure.ac b/configure.ac
index 01d9ded69b920..9b4c8833536ab 100644
--- a/configure.ac
+++ b/configure.ac
@@ -378,7 +378,6 @@ AC_CHECK_HEADERS(m4_normalize([
   pty.h
   pwd.h
   resolv.h
-  strings.h
   syslog.h
   sysexits.h
   sys/ioctl.h
@@ -579,7 +578,6 @@ AC_CHECK_FUNCS(m4_normalize([
   statfs
   statvfs
   std_syslog
-  strcasecmp
   strnlen
   strptime
   strtok_r
diff --git a/ext/mbstring/config.m4 b/ext/mbstring/config.m4
index 3030147028310..533d27ac68d87 100644
--- a/ext/mbstring/config.m4
+++ b/ext/mbstring/config.m4
@@ -52,10 +52,15 @@ AC_DEFUN([PHP_MBSTRING_SETUP_MBREGEX], [
   PHP_INSTALL_HEADERS([ext/mbstring], [php_mbregex.h php_onig_compat.h])
 ])
 
+dnl
+dnl PHP_MBSTRING_SETUP_LIBMBFL
+dnl
+dnl Configure bundled libmbfl. It is required and cannot be disabled.
+dnl
 AC_DEFUN([PHP_MBSTRING_SETUP_LIBMBFL], [
-  dnl
-  dnl Bundled libmbfl is required and can not be disabled
-  dnl
+  AC_CHECK_HEADERS([strings.h])
+  AC_CHECK_FUNCS([strcasecmp])
+
   PHP_MBSTRING_ADD_BUILD_DIR([libmbfl])
   PHP_MBSTRING_ADD_BUILD_DIR([libmbfl/mbfl])
   PHP_MBSTRING_ADD_BUILD_DIR([libmbfl/filters])
@@ -144,7 +149,6 @@ if test "$PHP_MBSTRING" != "no"; then
 
   AS_VAR_IF([PHP_MBREGEX], [yes], [PHP_MBSTRING_SETUP_MBREGEX])
 
-  dnl libmbfl is required
   PHP_MBSTRING_SETUP_LIBMBFL
 
   PHP_NEW_EXTENSION([mbstring],
@@ -159,18 +163,15 @@ if test "$PHP_MBSTRING" != "no"; then
   done
 
   for dir in $PHP_MBSTRING_EXTRA_INCLUDES; do
-    PHP_ADD_INCLUDE([$ext_srcdir/$dir])
     PHP_ADD_INCLUDE([$ext_builddir/$dir])
+    PHP_ADD_INCLUDE([$ext_srcdir/$dir])
   done
 
-  out="php_config.h"
-
-  if test "$ext_shared" != "no" && test -f "$ext_builddir/config.h.in"; then
-    out="$abs_builddir/config.h"
-  fi
-
   cat > $ext_builddir/libmbfl/config.h <<EOF
-#include "$out"
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+#include <php_config.h>
 EOF
 
   PHP_INSTALL_HEADERS([ext/mbstring], [mbstring.h])
diff --git a/ext/mbstring/config.w32 b/ext/mbstring/config.w32
index 070d3d73137ac..dfe5beb589ada 100644
--- a/ext/mbstring/config.w32
+++ b/ext/mbstring/config.w32
@@ -12,12 +12,15 @@ if (PHP_MBSTRING != "no") {
 		STDOUT.WriteLine("Using bundled libmbfl...");
 
 		ADD_FLAG("CFLAGS_MBSTRING", "-Iext/mbstring -Iext/mbstring/libmbfl -Iext/mbstring/libmbfl/mbfl \
-			/D HAVE_STRICMP /D MBFL_DLL_EXPORT=1 /DZEND_ENABLE_STATIC_TSRMLS_CACHE=1")
+			/D MBFL_DLL_EXPORT=1 /DZEND_ENABLE_STATIC_TSRMLS_CACHE=1")
 
 		ADD_FLAG("CFLAGS_BD_EXT_MBSTRING", "/utf-8")
 
-		FSO.CopyFile("ext\\mbstring\\libmbfl\\config.h.w32",
-			"ext\\mbstring\\libmbfl\\config.h", true);
+		var mbfl_config = FSO.CreateTextFile("ext/mbstring/libmbfl/config.h", true);
+		mbfl_config.WriteLine("#define HAVE_STRCASECMP 1");
+		mbfl_config.WriteLine("#define HAVE_STRICMP 1");
+		mbfl_config.WriteLine("#include <config.w32.h>");
+		mbfl_config.Close();
 
 		ADD_SOURCES("ext/mbstring/libmbfl/filters", "html_entities.c \
 			mbfilter_7bit.c mbfilter_base64.c \
diff --git a/ext/mbstring/libmbfl/config.h.w32 b/ext/mbstring/libmbfl/config.h.w32
deleted file mode 100644
index 6d05bfc67c363..0000000000000
--- a/ext/mbstring/libmbfl/config.h.w32
+++ /dev/null
@@ -1 +0,0 @@
-#define HAVE_STRICMP 1
diff --git a/win32/build/config.w32.h.in b/win32/build/config.w32.h.in
index c3d2fba966bc2..2de2ad47ac19a 100644
--- a/win32/build/config.w32.h.in
+++ b/win32/build/config.w32.h.in
@@ -61,7 +61,6 @@
 #define HAVE_STRUCT_STAT_ST_RDEV 1
 #define HAVE_GETLOGIN 1
 #define HAVE_SHUTDOWN 1
-#define HAVE_STRCASECMP 1
 #define HAVE_UTIME 1
 #undef HAVE_DIRENT_H
 #define HAVE_FCNTL_H 1