
Implement the `mysubstr()` function, which extracts a substring of a specified length from a string. It takes two arguments (string and length) and returns a substring starting from the first character:

Call example:

```php
<?php

$str = 'If I look back I am lost';
mysubstr($str, 1); // 'I'
mysubstr($str, 7); // 'If I lo'
```

https://replit.com/@hexlet/php-basics-edge-cases-mysubstr

Use the same approach as you did for the function to flip the string from the lesson: assemble the resulting string in a loop by looping through the initial string up to a certain point.
