Improved named arguments

A better example for ``substr_compare`` http://3v4l.org/5BmHs:

```php
<?php
var_dump(substr_compare('abcd', 'bc', 1, null, false));
var_dump(substr_compare('abcd', 'bc', 1));

var_dump(substr_compare('abcd', 'bcd', 1, null, false));
var_dump(substr_compare('abcd', 'bcd', 1));

var_dump(substr_compare('abcd', 'BCD', 1, null, true));
var_dump(substr_compare('abcd', 'BCD', 1, 3, true));
```
Output for hhvm-3.0.1 - 3.3.0:

```
Warning: The length must be greater than zero in /in/5BmHs on line 2
bool(false)
int(1)

Warning: The length must be greater than zero in /in/5BmHs on line 5
bool(false)
int(0)

Warning: The length must be greater than zero in /in/5BmHs on line 8
bool(false)
int(0)
```

Output for 5.5.12 - 5.6.1, php7@20140507 - 20141001:
```
int(0)
int(1)
int(0)
int(0)
int(0)
int(0)
```

Output for 5.5.11:
```
int(0)
int(0)
int(0)
int(0)
int(0)
int(0)
```

Output for 5.1.5 - 5.5.10:
```
Warning: substr_compare(): The length must be greater than zero in /in/5BmHs on line 2
bool(false)
int(1)

Warning: substr_compare(): The length must be greater than zero in /in/5BmHs on line 5
bool(false)
int(0)

Warning: substr_compare(): The length must be greater than zero in /in/5BmHs on line 8
bool(false)
int(0)
```