(PHP 7 >= 7.2.0, PHP 8)
sodium_crypto_box_publickey_from_secretkey — Calculate the public key from a secret key
Given a secret key, calculate the corresponding public key.
This only works with the type of keys intended for use with crypto_box() (which uses Elliptic Curve Diffie-Hellman over the Montgomery curve, Curve25519; abbreviated as X25519), not crypto_sign() (which uses Edwards Digital Signature Algorithm over the Edwards Curve with the corresponding paramaters; abbreviated Ed25519).
secret_keyX25519 secret key
X25519 public key.