(PHP 7 >= 7.2.0, PHP 8)
sodium_crypto_box_secretkey — Extracts the secret key from a crypto_box keypair
$key_pair): stringGiven a keypair, fetch only the secret key.
key_pairA keypair, such as one generated by sodium_crypto_box_keypair() or sodium_crypto_box_seed_keypair()
X25519 secret key.