Thanks for the swift reply @michaeld. Will give it a go later today 
For anyone interested and doing this in PHP, I used a composer library called base62 by tuupola.
This is the code I used:
<?php
$base62 = new Tuupola\Base62(["characters" => Tuupola\Base62::INVERTED]);
/** Set the original file name, excluding any file extensions */
$s = "r3AYqESanERjladb4vBB7VsMBm6";
/** Decode, convert to hex */
$decoded = $base62->decode($s);
/** Expected result: bda2c513e1da04f7b4e99230851ea2aafeb8cc4e */
echo bin2hex($decoded);