I compiled PHP 4.0.4pl1 with --bz2, using bzip 1.0.1. Running bzcompress() on a string always returns a string of the same size. The data in the returned string appears to be correctlty compressed though, as it uncompresses correctly when passed back to bzdecompress(). It appears that the first part of the returned string contains the correctly compressed result, and that the returned string is then padded with nulls out to the size of the original string. The gzcompress() function does not behave this way.

<?
$str = "[...some really huge string...]";
echo
  strlen($str) . " " .
  strlen(gzcompress($str,9)) . " " .
  strlen(bzcompress($str,9));
?>

This generates the output: 14092 2348 14092