Fix typo

Author: petk

docs/differences.md

@@ -38,7 +38,7 @@ build system:
   the cross-compiling emulator.
 
 * The installation include directory (`/usr/local/include/php`) can be adjusted
-  with `PHP_INSTALL_PREFIX` variable to support multiple PHP versions. For
+  with `PHP_INCLUDE_PREFIX` variable to support multiple PHP versions. For
   example, `/usr/local/include/php/8.4`.
 
 ## Behavior

docs/php-installation.md

@@ -230,7 +230,7 @@ prefix will not be taken into account.
 > ```sh
 > <CMAKE_INSTALL_PREFIX>          # /usr/local
 > └─ <CMAKE_INSTALL_INCLUDEDIR>   # └─ include
->    └─ <PHP_INSTALL_PREFIX>      #    └─ php
+>    └─ <PHP_INCLUDE_PREFIX>      #    └─ php
 >       ├─ ext                    #       ├─ ext
 >       ├─ main                   #       ├─ main
 >       ├─ sapi                   #       ├─ sapi