Added PHP!Anuna

Webshells/WShell_PHP_Anuna.yar

+/*
+    I first found this in May 2016, appeared in every PHP file on the
+    server, cleaned it with `sed` and regex magic. Second time was
+    in June 2016, same decoded content, different encoding/naming.
+
+    https://www.symantec.com/security_response/writeup.jsp?docid=2015-111911-4342-99
+*/
+rule php_anuna
+{
+    meta:
+        author      = "Vlad https://github.com/vlad-s"
+        date        = "2016/07/18"
+        description = "Catches a PHP Trojan"
+    strings:
+        $a = /<\?php \$[a-z]+ = '/
+        $b = /\$[a-z]+=explode\(chr\(\([0-9]+[-+][0-9]+\)\)/
+        $c = /\$[a-z]+=\([0-9]+[-+][0-9]+\)/
+        $d = /if \(!function_exists\('[a-z]+'\)\)/
+    condition:
+        all of them
+}