So, from the private key above, using this library, you need to 'BIP32::import()' the extended key in order to access the 'key' parameter. This is the hex private key, so you need to call BitcoinLib::private_key_to_WIF($imported_bip32key['key'], '6f', TRUE) - 6f for testnet; TRUE to return a WIF for a compressed pubkey address.
You can import WIF keys into bitcoind then!
$label= 'test import key';
$decoded_key = BIP32::import($bitcoinprivkey);
$private_key_hex = $decoded_key['key']; // hexadecimal
$wif = BitcoinLib::private_key_to_WIF(); // base58 wallet import format
?>
If you do:
print_r(BIP32::import('tprv8baY42qkWkxUTXPRrkfNx1iEg3GxVJBeGRrbFWorwjao6a4KFtycUcY2Rjp5tXvGeQwvXH3s7KA HCKkueb8bjPnHFGZEfc3eYc6v437192G'));
You'll see all the data which is compacted into BIP32 keys, making them longer, but assisting with the deterministic algorithm used to generate them.