Document bareword string deprecation in php/base64

modules/encoders/php/base64.rb

@@ -38,6 +38,8 @@ class MetasploitModule < Msf::Encoder
     # characters, only part of the payload gets unencoded on the victim,
     # presumably due to a limitation in PHP identifier name lengths, so we
     # break the encoded payload into roughly 900-byte chunks.
+    #
+    # https://wiki.php.net/rfc/deprecate-bareword-strings
 
     b64 = Rex::Text.encode_base64(buf)