Prevent PHP from parsing your MySQL credentials

## Problem

If you use a $-character in your MySQL username/password/dbname, it won't be escaped before writing it into the configuration file and because of the fact, that Baikal currently uses double-quotes for strings, PHP parses the string, finds a variable and tries to resolve it which cannot be found. This causes a "undefined variable $bug" E_NOTICE.

### Installation

````
[...]
MySQL password: this_is_a_$bug
[...]
````

### Configuration (config.system.php)

````php
// ...

define("PROJECT_DB_MYSQL_PASSWORD", "this_is_a_$bug");

// ...
````

### Error

> Notice: Undefined variable: bug in [...]/Specific/config.system.php on line 66