<?php
$txt = "5_hundred_thusands_text_charactersPART2fivehundredthusandstextcharactersagain";
//$arrParts = explode('PART2', $txt);
$arrSplitItems = str_split($arrParts[1], 10);
PD9waHAKJHR4dCA9ICI1X2h1bmRyZWRfdGh1c2FuZHNfdGV4dF9jaGFyYWN0ZXJzUEFSVDJmaXZlaHVuZHJlZHRodXNhbmRzdGV4dGNoYXJhY3RlcnNhZ2FpbiI7CiRhcnJQYXJ0cyA9IHByZWdfc3BsaXQoIi9QQVJUMi8iLCAkdHh0KTsKLy8kYXJyUGFydHMgPSBleHBsb2RlKCdQQVJUMicsICR0eHQpOwoKJGFyclNwbGl0SXRlbXMgPSBzdHJfc3BsaXQoJGFyclBhcnRzWzFdLCAxMCk7CmVjaG8gaW1wbG9kZSgnLCcsICRhcnJTcGxpdEl0ZW1zKTs=