The task of a job interview at one major Swedish site

I am a PHP-Developer, I live in Stockholm. Recently I was interviewed in one large Swedish site (more than a billion page views per month). The interview was conducted by 2 programmers from this company. At a certain point, one of them took out a piece of paper and said that they were offering me to solve a small problem (right there on paper, without a computer). And that I have 10 minutes. They asked to comment on each step in the same way.

I will say right away that I did not solve it. At first everything seemed to be simple, and then ... So, I left the meeting not saltyly slurping. For my part, she remained unresolved.

Why publish this? Firstly, it might be useful to someone as a good test for hired developers; secondly, someone, if he encounters something similar, will already be full of knowledge; thirdly, can someone put the right decision in the comments?

Below is the task itself. I leave everything in the original as it was.

PHP assignment
Write a function, read_conf ($ filename), that converts the configuration below into a multidimensional array.

The configuration is divided up in rows and each row is divided up by key and value. The key can be multidimensional, and can be from 1 ... N, in the example below we only have 4 levels, but the solution should be able to work even when adding another row with more key levels: eg. session.save.db.master.host = 10.0.0.1

=== config.txt ===
id = www
session.timeout = 120
session.server.0.host = 127.0.0.1
session.server.0.port = 1111
session.server.0.id = session1
session.server.1.host = 127.0.0.1
session.server.1.port = 1111
session.server.1.id = session2
image.width = 640
image.height = 480
image.watermark.small = wsmall.png
image.watermark.normal = wnormal.png

=== code ===
$ res = read_conf (" config.txt ");
var_dump ($ res);
?>

=== output ===
array(3) {
["id"]=>strong(3) "www"
["session"]=>array(2) {
["timeout"]=>string(3) "120"
["server"]=>array(2) {
[0]=>
array(3) {
["host"]=>
string(9) "127.0.0.1"
["post"]=>
string(4) "1111"
["id"]=>
string(8) "session1"
}
[1]=>
array(3) {
["host"]=>
string(9) "127.0.0.1"
["port"]=>
string(4) "1111"
["id"]=>
string(8) "session2"
}
}
}
["image"]=>
array(3) {
["width"]=>
string(3) "640"
["height"]=>
string(3) "480"
["watermark"]=>
array(2) {
["small"]=>
string(10) "wsmall.png"
["normal"]=>
string(11) "wnormal.png"
}
}
}