Requires BC. Convert info_hash from short versions (deprecated) to 40-char length ones.

gistfile1.php

<?php
function convert_base($numberInput, $fromBaseInput, $toBaseInput) {
  if ($fromBaseInput == $toBaseInput) return $numberInput;
	
	$fromBase = str_split($fromBaseInput,1);
	$toBase = str_split($toBaseInput,1);
	$number = str_split($numberInput,1);
	$fromLen = strlen($fromBaseInput);
	$toLen = strlen($toBaseInput);
	$numberLen = strlen($numberInput);
	$retval = '';
	
	if($toBaseInput == '0123456789') {
		$retval = 0;
		
		for($i = 1; $i <= $numberLen; $i++) {
			$retval = bcadd($retval, bcmul(array_search($number[$i-1], $fromBase), bcpow($fromLen, $numberLen-$i)));
		}
		
		return $retval;
	}
	
	if($fromBaseInput != '0123456789') {
		$base10 = convert_base($numberInput, $fromBaseInput, '0123456789');
	} else {
		$base10 = $numberInput;
	}
	
	if ($base10 < strlen($toBaseInput)) {
		return $toBase[$base10];
	}
	
	while($base10 != '0') {
		$retval = $toBase[bcmod($base10,$toLen)].$retval;
		$base10 = bcdiv($base10,$toLen,0);
	}
	
	return $retval;
}

function convert_info_hash($input) {
	$result = convert_base($input, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ234567', '0123456789ABCDEF');
	$result = str_pad($result, 40, '0', STR_PAD_LEFT);
	
	return $result;
}