diff --git a/build/php.m4 b/build/php.m4
index aa49766fedd7..aeb68bb92b95 100644
--- a/build/php.m4
+++ b/build/php.m4
@@ -2396,6 +2396,9 @@ AC_DEFUN([PHP_CHECK_STDINT_TYPES], [
   AC_CHECK_SIZEOF([long long])
   AC_CHECK_SIZEOF([size_t])
   AC_CHECK_SIZEOF([off_t])
+
+  AS_IF([test $ac_cv_sizeof_long = 4],
+    AC_MSG_WARN([Size of long type is 4 bytes. 32 bit builds are deprecated and will no longer be supported in PHP-9]))
 ])
 
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