How do I encrypt a password in Wordpress using Wordpress salt?

Use wp_hash_password() to hash your password.

Don't use mysql_* functions as they were deprecated in PHP 5.5.0, and were removed in PHP 7.0.0.

Use the WPDB Class instead.

$wpdb->query(     $wpdb->prepare(         "        INSERT INTO  mytable_one        ( fname, lname, user, pass )        VALUES ( %s, %s, %s, %s )        ",           $firstname,           $lastname,           $username,           wp_hash_password( $password )        ));

you should use bcrypt for securing passwords

here is an example class that use for my projects.

<?php    // How to use it    // $bcrypt = new Bcrypt(15);    // $hash = $bcrypt->hash('password');    // $isGood = $bcrypt->verify('password', $hash);    class Bcrypt {      private $rounds;      public function __construct($rounds = 12) {        if(CRYPT_BLOWFISH != 1) {          throw new Exception("bcrypt not supported in this installation. See http://php.net/crypt");        }        $this->rounds = $rounds;      }      public function hash($input) {        $hash = crypt($input, $this->getSalt());        if(strlen($hash) > 13)          return $hash;        return false;      }      public function verify($input, $existingHash) {        $hash = crypt($input, $existingHash);        return $hash === $existingHash;      }      private function getSalt() {        $salt = sprintf('$2a$%02d$', $this->rounds);        $bytes = $this->getRandomBytes(16);        $salt .= $this->encodeBytes($bytes);        return $salt;      }      private $randomState;      private function getRandomBytes($count) {        $bytes = '';        if(function_exists('openssl_random_pseudo_bytes') &&            (strtoupper(substr(PHP_OS, 0, 3)) !== 'WIN')) { // OpenSSL slow on Win          $bytes = openssl_random_pseudo_bytes($count);        }        if($bytes === '' && is_readable('/dev/urandom') &&           ($hRand = @fopen('/dev/urandom', 'rb')) !== FALSE) {          $bytes = fread($hRand, $count);          fclose($hRand);        }        if(strlen($bytes) < $count) {          $bytes = '';          if($this->randomState === null) {            $this->randomState = microtime();            if(function_exists('getmypid')) {              $this->randomState .= getmypid();            }          }          for($i = 0; $i < $count; $i += 16) {            $this->randomState = md5(microtime() . $this->randomState);            if (PHP_VERSION >= '5') {              $bytes .= md5($this->randomState, true);            } else {              $bytes .= pack('H*', md5($this->randomState));            }          }          $bytes = substr($bytes, 0, $count);        }        return $bytes;      }      private function encodeBytes($input) {        // The following is code from the PHP Password Hashing Framework        $itoa64 = './ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';        $output = '';        $i = 0;        do {          $c1 = ord($input[$i++]);          $output .= $itoa64[$c1 >> 2];          $c1 = ($c1 & 0x03) << 4;          if ($i >= 16) {            $output .= $itoa64[$c1];            break;          }          $c2 = ord($input[$i++]);          $c1 |= $c2 >> 4;          $output .= $itoa64[$c1];          $c1 = ($c2 & 0x0f) << 2;          $c2 = ord($input[$i++]);          $c1 |= $c2 >> 6;          $output .= $itoa64[$c1];          $output .= $itoa64[$c2 & 0x3f];        } while (1);        return $output;      }    }    ?>