Description
bool 
openssl_pkey_export_to_file ( mixed key, string outfilename [, string passphrase [, array configargs]] )
     openssl_pkey_export_to_file() saves an ascii-armoured
     (PEM encoded) rendition of key into the file named
     by outfilename.  The key can be optionally
     protected by a passphrase.
     configargs can be used to fine-tune the export
     process by specifying and/or overriding options for the openssl
     configuration file.  See openssl_csr_new() for more
     information about configargs.
     Returns TRUE on success or FALSE on failure.
    
Note: 
  You need to have a valid openssl.cnf installed for
  this function to operate correctly.
  See the notes under the installation
  section for more information.