CS: PHP keywords MUST be in lower case

A simple typo  can create bitwise operators (`|` and `&`) from `||` and `&&`. You won't get any warning at all, because `|` and `&` are valid operators. You can not make that mistake by using `and` and `or`. Global constants can not be used that way and PHP will use the operator instead of the constant:
```php
<?php
define('AND', 'myValue');
var_dump(false AND true); // false
```
Variables with names `$and` and `$or` will trigger notices (undefined var).

Looking at what can go wrong, I think `&&` and `||` are more likely to cause problems than `and` and `or`.