diff --git a/ext/iconv/config.m4 b/ext/iconv/config.m4
index 4d0dc36489833..e86f3535a97c4 100644
--- a/ext/iconv/config.m4
+++ b/ext/iconv/config.m4
@@ -83,6 +83,20 @@ int main(void) {
   AS_VAR_IF([php_cv_iconv_errno], [yes],,
     [AC_MSG_FAILURE([The iconv check failed, 'errno' is missing.])])
 
+  dnl iconv on some platforms (NetBSD pre-10, Solaris) may have a non-standard
+  dnl const input parameter; libiconv may imitate this on those platforms.
+  AC_CACHE_CHECK([if iconv input parameter is const (non-standard)], [php_cv_iconv_const],
+    [AC_COMPILE_IFELSE([AC_LANG_SOURCE([
+#include <iconv.h>
+
+size_t iconv(iconv_t cd, const char **src, size_t *srcleft, char **dst, size_t *dstleft);
+    ])],
+    [php_cv_iconv_const=const],
+    [php_cv_iconv_const=non-const])])
+  dnl non-const is just used for display, set it back
+  AS_VAR_IF([php_cv_iconv_const], [non-const],
+    [php_cv_iconv_const=])
+
   AC_CACHE_CHECK([if iconv supports //IGNORE], [php_cv_iconv_ignore],
     [AC_RUN_IFELSE([AC_LANG_SOURCE([
 #include <iconv.h>
@@ -120,7 +134,7 @@ int main(void) {
   PHP_NEW_EXTENSION([iconv],
     [iconv.c],
     [$ext_shared],,
-    [-DZEND_ENABLE_STATIC_TSRMLS_CACHE=1])
+    [-DZEND_ENABLE_STATIC_TSRMLS_CACHE=1 -DICONV_CONST=$php_cv_iconv_const])
   PHP_SUBST([ICONV_SHARED_LIBADD])
   PHP_INSTALL_HEADERS([ext/iconv], [php_iconv.h])
 fi
diff --git a/ext/iconv/iconv.c b/ext/iconv/iconv.c
index 7bf0c6d23401c..7c812f5af44b4 100644
--- a/ext/iconv/iconv.c
+++ b/ext/iconv/iconv.c
@@ -43,11 +43,11 @@
 #undef iconv
 #endif
 
-#if defined(__NetBSD__) || (defined(__sun) && defined(__SVR4))
-// unfortunately, netbsd has still the old non posix conformant signature
-// libiconv tends to match the eventual system's iconv too.
-#define ICONV_CONST const
-#else
+/* iconv can have different constiness for src on some platforms;
+ * this is explained in config.m4. On Windows, it's always non-const,
+ * but it can be awkward to set that on the command line. Do it here.
+ */
+#ifndef ICONV_CONST
 #define ICONV_CONST
 #endif