diff --git a/config.m4 b/config.m4
index f07e080..8f000c2 100644
--- a/config.m4
+++ b/config.m4
@@ -16,7 +16,7 @@ if test "$PHP_ARMA" != "no"; then
     PCH_FILE='./build/armadillo.gch'
     # Note that '-D' option is valid when compiling PCH header, but not when using it.
     PCH_COMPILE_FLAGS=(-std=c++17 -xc++-header -DARMA_DONT_PRINT_ERRORS -fPIC -g)
-    if test -z "$PHP_DEBUG"; then
+    if test "$PHP_DEBUG" != "0"; then
       PCH_COMPILE_FLAGS+=(-O0)
     else
       PCH_COMPILE_FLAGS+=(-O2)