Read last chunk of concatenated gzip file in PHP

gzfile_get_last_chunk_of_concatenated_file.php

<?php

declare(strict_types=1);

/**
 * https://gist.github.com/fox34/2503bc11cfc1f1001a919ac655e48706
 *
 * Multiple gzip files can simply be concatenated. This function tries to extract the last chunk of a concatenated gzip file.
 * Works by searching for the magic number 1f 8b
 * Poor performance because of backtracking and multiple re-reads
 * Thus, use only for smaller chunks of locally available data.
 * Limited to maximum 1MB of compressed data (uncompressed data may be larger)
 */
function gzfile_get_last_chunk_of_concatenated_file(string $file, int $readLimit = 1000000) : string
{
    // Limit to 1MB
    $readLimit = min($readLimit, 1e6);
    
    $fp = fopen($file, 'rb');
    if ($fp === false) {
        throw new \Exception('Could not read file.');
    }
    
    fseek($fp, -2, SEEK_END);
    $gzdata = '';
    $data = '';
    $counter = 0;
    
    // Read chunks of 2 bytes and compare with magic number
    while (($seq = fread($fp, 2)) && $counter++ < $readLimit) {
        
        // magic number not matched
        if (bin2hex($seq) !== '1f8b') {
            fseek($fp, -3, SEEK_CUR);
            continue;
        }
        
        $pos = ftell($fp);
        $gzdata = $seq;
        
        // Read all remaining data
        while ($chunk = fread($fp, 1024)) {
            $gzdata .= $chunk;
        }
        
        // Try decoding data
        $data = @gzdecode($gzdata);
        
        // Could not decode data. Maybe magic number appeared inside compressed content?
        if ($data === false) {
            $data = '';
            fseek($fp, $pos - 3);
        } else {
            fclose($fp);
            return $data;
        }
    }
    fclose($fp);
    
    throw new \Exception('Valid chunk not found within provided size limit.');
}