( string filename, string mode [, int use_include_path] )
Opens a gzip (.gz) file for reading or writing.
gzopen() can be used to read a file which is
not in gzip format; in this case gzread() will
directly read from the file without decompression.
Returns a file pointer to the file opened, after that, everything you read
from this file descriptor will be transparently decompressed and what you
write gets compressed.
If the open fails, the function returns FALSE.