ICU 74 headers require C++17.
--- a/ext/intl/config.m4
+++ b/ext/intl/config.m4
@@ -83,7 +83,15 @@
     breakiterator/codepointiterator_methods.cpp"
 
   PHP_REQUIRE_CXX()
-  PHP_CXX_COMPILE_STDCXX(11, mandatory, PHP_INTL_STDCXX)
+
+  AC_MSG_CHECKING([if intl requires -std=gnu++17])
+  AS_IF([$PKG_CONFIG icu-uc --atleast-version=74],[
+    AC_MSG_RESULT([yes])
+    PHP_CXX_COMPILE_STDCXX(17, mandatory, PHP_INTL_STDCXX)
+  ],[
+    AC_MSG_RESULT([no])
+    PHP_CXX_COMPILE_STDCXX(11, mandatory, PHP_INTL_STDCXX)
+  ])
   PHP_INTL_CXX_FLAGS="$INTL_COMMON_FLAGS $PHP_INTL_STDCXX $ICU_CXXFLAGS"
   if test "$ext_shared" = "no"; then
     PHP_ADD_SOURCES(PHP_EXT_DIR(intl), $PHP_INTL_CXX_SOURCES, $PHP_INTL_CXX_FLAGS)