Php nested ob_start explanation

ob_start_test.php

<?php  
// without OB
echo 'before_ob' . PHP_EOL; 
ob_start(); // first level begins

// this line will be in $second
echo 'second_ob' . PHP_EOL; 
ob_start(); // second level begins

// these 2 lines will be in $first
echo 'third_ob' . PHP_EOL; 
echo '------' . PHP_EOL;  
$first = ob_get_clean() . 1; 

// this line will be in second
// but w/o previous 2 lines
echo '------' . PHP_EOL;  
$second = ob_get_clean() . 2; 

// this will output $first consisting of 2 lines.
// and $second without them.
var_dump($first, $second);
/**
Output:

before_ob
string(17) "third_ob
------
1"
string(18) "second_ob
------
2"

testscript: http://biozshock.com/test_ob.php
**/