Convert from Mozilla's LZ4 format to LZ4 v1.3

convert_lz4_header.php

<?php

/**
 * Convert from Mozilla's LZ4 format to LZ4 v1.3
 *
 * @link https://searchfox.org/mozilla-central/source/toolkit/components/lz4/lz4.js
 * @link https://lz4.software.informer.com/1.3/
 *
 * @param  string $input File content in Mozilla's LZ4 format
 * @return string        File content converted to LZ4 v1.3
 *
 * @throws \RuntimeException If input data appears not to be of Mozilla's LZ4 format
 */
function convert_lz4_header($input) {

	$moz_header = substr($input, 0, 12);
	$compressed_data = substr($input, 12);

	if (substr($moz_header, 0, 8) !== "mozLz40\0") {
		throw new \RuntimeException('Wrong magic number');
	}

	$new_header = pack('V', 0x184C2103) // magic number LZ4 v1.0-1.3, little endian
	            . substr($moz_header, 8, 4) // size of decompressed file, uint32 little endian
	            . hex2bin('0000004D') // I don't know what this is, but it never changes
	            . pack('V', strlen($compressed_data)); // size of compressed data (without header), uint32 little endian

	return $new_header . $compressed_data;
}

See: https://bugzilla.mozilla.org/show_bug.cgi?id=1209390