Api.php
private function encrypt_3DES($merchantOrder, $key)
    {

        $ciphertext = null;
        if ( function_exists( 'mcrypt_encrypt' ) && version_compare(phpversion(), '7.1', '<')  ) {
            // default IV
            $bytes = array(0,0,0,0,0,0,0,0); //byte [] IV = {0, 0, 0, 0, 0, 0, 0, 0}
            $iv = implode(array_map("chr", $bytes));
            $ciphertext = mcrypt_encrypt(MCRYPT_3DES, $key, $merchantOrder, MCRYPT_MODE_CBC, $iv);

        } elseif(function_exists( 'openssl_encrypt' ) && version_compare(phpversion(), '7.1', '>=') ) {
            $l = ceil(strlen($merchantOrder) / 8) * 8;
            $ciphertext = substr(openssl_encrypt($merchantOrder . str_repeat("\0", $l - strlen($merchantOrder)), 'des-ede3-cbc', $key, OPENSSL_RAW_DATA, "\0\0\0\0\0\0\0\0"), 0, $l);
        }

        return $ciphertext;
    }
private function encrypt_3DES($merchantOrder, $key)
    {

        $ciphertext = null;
        if ( function_exists( 'mcrypt_encrypt' ) && version_compare(phpversion(), '7.1', '<')  ) {
            // default IV
            $bytes = array(0,0,0,0,0,0,0,0); //byte [] IV = {0, 0, 0, 0, 0, 0, 0, 0}
            $iv = implode(array_map("chr", $bytes));
            $ciphertext = mcrypt_encrypt(MCRYPT_3DES, $key, $merchantOrder, MCRYPT_MODE_CBC, $iv);

        } elseif(function_exists( 'openssl_encrypt' ) && version_compare(phpversion(), '7.1', '>=') ) {
            $l = ceil(strlen($merchantOrder) / 8) * 8;
            $ciphertext = substr(openssl_encrypt($merchantOrder . str_repeat("\0", $l - strlen($merchantOrder)), 'des-ede3-cbc', $key, OPENSSL_RAW_DATA, "\0\0\0\0\0\0\0\0"), 0, $l);
        }

        return $ciphertext;
    }
3DES Function provided by Redsys

@param string $merchantOrder
@param string $key

@return string