You can't use this function to export a public key
Warning: openssl_pkey_export_to_file(): supplied key param is a public key
(PHP 4 >= 4.2.0, PHP 5)
openssl_pkey_export_to_file — Gets an exportable representation of a key into a file
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.
Path to the output file.
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.