PHP Base-62 encoder/decoder

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<?php
/**
 * This is an example of a practical encoder and decoder for base-62 data in PHP
 * It differs from the majority of examples in that it's fast enough for moderate data sizes, unlike multiprecision converters
 * To be practical, base-62 encoding needs to use internal chunking and padding because base-62 does not fit exactly into any integral number of bits
 * This means the output is not quite compatible with multiprecision conversions,
 * but the encoded data retains all the desirable properties of base-62, so (unlike any base-64 encoding) it's URL, DNS, email address and pathname safe
 * @author     Marcus Bointon <marcus@synchromedia.co.uk>
 * @copyright  2011 Marcus Bointon
 * @license    http://www.opensource.org/licenses/mit-license.html MIT License
 */
$data = '';
//Make a nice 1M chunk of randomish binary data
for($q=0; $q < 64000; $q++) {
	$data .=md5($q, true);
}
echo "data size: ".strlen($data)."\n";
$zz = microtime(true);
$encoded = base62encode($data);
echo "enc = ".(microtime(true) - $zz)."sec\n";
$zz = microtime(true);
$decoded = base62decode($encoded);
echo "dec = ".(microtime(true) - $zz)."sec\n";
if ($data == $decoded) {
	echo "match\n";
} else {
	echo "fail\n";
}

/**
 * Encode arbitrary data into base-62
 * Note that because base-62 encodes slightly less than 6 bits per character (actually 5.95419631038688), there is some wastage
 * In order to make this practical, we chunk in groups of up to 8 input chars, which give up to 11 output chars
 * with a wastage of up to 4 bits per chunk, so while the output is not quite as space efficient as a
 * true multiprecision conversion, it's orders of magnitude faster
 * Note that the output of this function is not compatible with that of a multiprecision conversion, but it's a practical encoding implementation
 * The encoding overhead tends towards 37.5% with this chunk size; bigger chunk sizes can be slightly more space efficient, but may be slower
 * Base-64 doesn't suffer this problem because it fits into exactly 6 bits, so it generates the same results as a multiprecision conversion
 * Requires PHP 5.3.2 and gmp 4.2.0
 * @param string $data Binary data to encode
 * @return string Base-62 encoded text (not chunked or split)
 */
function base62encode($data) {
	$outstring = '';
	$l = strlen($data);
	for ($i = 0; $i < $l; $i += 8) {
		$chunk = substr($data, $i, 8);
		$outlen = ceil((strlen($chunk) * 8)/6); //8bit/char in, 6bits/char out, round up
		$x = bin2hex($chunk);  //gmp won't convert from binary, so go via hex
		$w = gmp_strval(gmp_init(ltrim($x, '0'), 16), 62); //gmp doesn't like leading 0s
		$pad = str_pad($w, $outlen, '0', STR_PAD_LEFT);
		$outstring .= $pad;
	}
	return $outstring;
}

/**
 * Decode base-62 encoded text into binary
 * Note that because base-62 encodes slightly less than 6 bits per character (actually 5.95419631038688), there is some wastage
 * In order to make this practical, we chunk in groups of up to 11 input chars, which give up to 8 output chars
 * with a wastage of up to 4 bits per chunk, so while the output is not quite as space efficient as a
 * true multiprecision conversion, it's orders of magnitude faster
 * Note that the input of this function is not compatible with that of a multiprecision conversion, but it's a practical encoding implementation
 * The encoding overhead tends towards 37.5% with this chunk size; bigger chunk sizes can be slightly more space efficient, but may be slower
 * Base-64 doesn't suffer this problem because it fits into exactly 6 bits, so it generates the same results as a multiprecision conversion
 * Requires PHP 5.3.2 and gmp 4.2.0
 * @param string $data Base-62 encoded text (not chunked or split)
 * @return string Decoded binary data
 */
function base62decode($data) {
	$outstring = '';
	$l = strlen($data);
	for ($i = 0; $i < $l; $i += 11) {
		$chunk = substr($data, $i, 11);
		$outlen = floor((strlen($chunk) * 6)/8); //6bit/char in, 8bits/char out, round down
		$y = gmp_strval(gmp_init(ltrim($chunk, '0'), 62), 16); //gmp doesn't like leading 0s
		$pad = str_pad($y, $outlen * 2, '0', STR_PAD_LEFT); //double output length as as we're going via hex (4bits/char)
		$outstring .= pack('H*', $pad); //same as hex2bin
	}
	return $outstring;
}
?>

PHP Fatal error: Call to undefined function gmp_init() in Psy Shell code on line 8 ?

install gmp module