Regexes are binary-safe. However you might be better off with strpos
.
$magicpos = strpos($downloaded_data,"\x1a\x09\x01");
That assumes the magic number is 0x1A 0x09 0x01 - you can replace it with whatever the number actually is. Then:
$archive = substr($downloaded_data,$magicpos);
This will get the archive data from the magic number (included) onwards.