diff --git a/free-programming-books.md b/free-programming-books.md
index 413e9a24fb243bb40b88d818c3845c4550b7912b..0d162544ea87b3fc2653e0b6919f4d87d6530d40 100644
--- a/free-programming-books.md
+++ b/free-programming-books.md
@@ -1479,6 +1479,7 @@ See also [TeX](#tex)
 * [Practical PHP Programming](http://www.tuxradar.com/practicalphp) (wiki containing O'Reilly's *PHP In a Nutshell*)
 * [Practical PHP Testing](http://www.giorgiosironi.com/2009/12/practical-php-testing-is-here.html)
 * [Symfony2](http://symfony.com/doc/current/book/index.html)
+* [Using Libsodium in PHP Projects](https://paragonie.com/book/pecl-libsodium) - libsodium PHP extension for modern, secure, and fast cryptography
 
 
 ### PicoLisp