diff --git a/CMakeLists.txt b/CMakeLists.txt
index 487bb6e3..6e1a0b23 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -827,8 +827,8 @@ if (COOLPROP_PHP_MODULE)
     OUTPUT_VARIABLE php_config_includes
     RESULT_VARIABLE php_config_failed
     )
-  string(STRIP ${php_config_includes} php_config_includes)
-  string(REPLACE "-I" "" PHP_INCLUDES ${php_config_includes})
+  string(STRIP "${php_config_includes}" php_config_includes)
+  string(REPLACE "-I" "" PHP_INCLUDES "${php_config_includes}")
   SEPARATE_ARGUMENTS(PHP_INCLUDES)
 
   message(STATUS "php includes=${PHP_INCLUDES}")