(PHP 4 >= 4.2.0, PHP 5)
openssl_pkey_export_to_file — Gets an exportable representation of a key into a file
$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.  
  
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.
key
outfilenamePath to the output file.
passphrase
       The key can be optionally protected by a
       passphrase.
      
configargs
       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.