gzread() fails when continue reads in seperate php requests - php

Question

I have gz compressed file and uncompressing it to normal file so I use fwrite(). It works fine when do uncompress in a single PHP request.
Because of large compressed files, I do mind in PHP timeouts, I uncompress the file until 30 seconds and stop the process and store the current offset of gz file using gztell() then continue it form the next PHP request where I left.
I do gzseek() with current offset and continue the uncompression but gzread() continuously gives empty string
function gz_uncompress_file($source, $offset = 0){
$dest = str_replace('.gz', '', $source);
$fp_in = gzopen($source, 'rb');
if (empty($fp_in)) {
return 'Cannot open gzfile to uncompress sql';
}
$fp_out = fopen($dest, 'ab');
if (empty($fp_out)) {
return 'Cannot open temp file to uncompress sql';
}
gzseek($fp_in, $offset);
$break = false;
while (!gzeof($fp_in)){
$chunk_data = gzread($fp_in, 1024 * 512);
if (empty($chunk_data)) {
return 'empty string so stop the process';
}
fwrite($fp_out, $chunk_data);
//Clearning to save memory
unset($chunk_data);
if(!is_time_out()){
continue;
}
$break = true;
$offset = gztell($fp_in);
break;
}
fclose($fp_out);
gzclose($fp_in);
if ($break) {
//Saving offset
$this->set_option('sql_gz_uncompression_offset', $offset);
return 'continue_from_call';
}
echo "Un compression done";
}