Convert from Mozilla's LZ4 format to LZ4 v1.3 (tested with FF51 profile & lz4 v1.7.5)

convert_lz4_header.php

<?php

/**
 * Convert from Mozilla's LZ4 format to LZ4 v1.3
 *
 * @link https://dxr.mozilla.org/mozilla-central/source/toolkit/components/lz4/lz4.js
 * @link https://github.com/lz4/lz4
 * @link https://github.com/lz4/lz4/blob/master/doc/lz4_Frame_format.md#legacy-frame
 *
 * @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');
	}

	$lz4_header = pack('V', 0x184C2102) // legacy frame magic number LZ4 v1.0-1.3, uint32_t little endian
	            . pack('V', strlen($compressed_data)); // size of compressed data (without header), uint32_t little endian

	return $lz4_header . $compressed_data;
}

I can't decode the generator file with the current version of lz4 :/

Error 44 : Unrecognized header : file cannot be decoded