This worked for me.

$new_password = password_hash($old_password, PASSWORD_BCRYPT);

password_hash() creates a new password hash using a strong one-way hashing algorithm. password_hash() is compatible with crypt().
Therefore, password hashes created by crypt() can be used with password_hash().

PASSWORD_BCRYPT – Use the CRYPT_BLOWFISH algorithm to create the hash.
This will produce a standard crypt() compatible hash using the “$2y$” identifier. The result will always be a 60 character string, or FALSE on failure. Supported Options:

http://php.net/manual/en/function.password-hash.php