Improve modules/nops/php/generic.rb

```irb
irb(main):001> length = 10
=> 10
irb(main):002>  Array.new(length) { ["\t", " ", "\n", "\r"].sample }.join
=> "  \r\t\n\t\t\n\t\r"
irb(main):003>
```

Author: jvoisin

modules/nops/php/generic.rb

@@ -22,7 +22,6 @@ def initialize
 
   # Generate valid PHP code up to the requested length
   def generate_sled(length, opts = {})
-    # Default to just spaces for now
-    " " * length
+    Array.new(length) { ["\t", " ", "\n", "\r", ";"].sample }.join
   end
 end