Add a `concat` rule

This aims to catch things like:

```php
<?php
$a = $p800['y6dfe'][9].$p800['y6dfe'][70].$p800['y6dfe'][48];$p800[$p800['y6dfe'][55].$p800['y6dfe'][8].$p800['y6dfe'][30].$p800['y6dfe'][9];

1 files changed, 1 insertions, 0 deletions

diff --git a/php-malware-finder/malwares.yara b/php-malware-finder/malwares.yara
index bfc4d7a..64c327b 100644
--- a/php-malware-finder/malwares.yara
+++ b/php-malware-finder/malwares.yara
@@ -64,6 +64,7 @@ rule ObfuscatedPhp
         $c99_launcher = /;\$\w+\(\$\w+(,\s?\$\w+)+\);/  // http://bartblaze.blogspot.fr/2015/03/c99shell-not-dead.html
         $variable_variable = /\${\$[0-9a-zA-z]+}/
         $too_many_chr = /(chr\([\d]+\)\.){2,}?/
+        $concat = /(\$[^\n\r]+\.){5,}?/
 condition:
         any of them and not IsWhitelisted
 }