Since seek after the end is not considered an error, I doubt that "while (gzseek ($fh, $eof) == 0) $eof += $d;" will get into infinite loop.(PHP 4, PHP 5, PHP 7, PHP 8)
gzseek — Seek on a gz-file pointer
   Sets the file position indicator for the given file pointer to the
   given offset byte into the file stream. Equivalent to calling (in C)
   gzseek(zp, offset, SEEK_SET).
  
If the file is opened for reading, this function is emulated but can be extremely slow. If the file is opened for writing, only forward seeks are supported; gzseek() then compresses a sequence of zeroes up to the new starting position.
streamThe gz-file pointer. It must be valid, and must point to a file successfully opened by gzopen().
offsetThe seeked offset.
whence
       whence values are:
       
SEEK_SET - Set position equal to offset bytes.SEEK_CUR - Set position to current location plus offset.
       If whence is not specified, it is assumed to be
       SEEK_SET.
      
Upon success, returns 0; otherwise, returns -1. Note that seeking past EOF is not considered an error.
Приклад #1 gzseek() example
<?php
$gz = gzopen('somefile.gz', 'r');
gzseek($gz,2);
echo gzgetc($gz);
gzclose($gz);
?>
Since seek after the end is not considered an error, I doubt that "while (gzseek ($fh, $eof) == 0) $eof += $d;" will get into infinite loop.PHP/4.3.9
contrary to the notes, gzseek() returns -1 if I try to seek past the end of the file.  here is a function that will return the last seekable position, and put the file pointer there.
/** sets the file pointer at the end of the file
 *  and returns the number of bytes in the file.
 */
function gzend($fh)
{
   $d   = 1<<14;
   $eof = $d;
   while ( gzseek($fh, $eof) == 0 ) $eof += $d;
   while ( $d > 1 )
   {
      $d >>= 1;
      $eof += $d * (gzseek($fh, $eof)? -1 : 1);
   }
   return $eof;
}