Improved named arguments

This commit fixes a wrong exception message:
```
Unknown argument "case_sensitivity" for function "substr_compare".
```

Note: In documentation is mentioned: ``int substr_compare ( string $main_str , string $str , int $offset [, int $length [, bool $case_insensitivity = false ]] )``, but in reflection the last argument is named ``case_sensitivity``. This is really a mess!