src/XdrModel/Operation/Operation.php
public function toXdr()
    {
        $bytes = '';

        // Source Account
        $bytes .= XdrEncoder::optional($this->sourceAccount);

        // Type
        $bytes .= XdrEncoder::unsignedInteger($this->type);

        return $bytes;
    }
public function toXdr()
    {
        $bytes = '';

        // Source Account
        $bytes .= XdrEncoder::optional($this->sourceAccount);

        // Type
        $bytes .= XdrEncoder::unsignedInteger($this->type);

        return $bytes;
    }
Child classes MUST call this method to get the header for the operation
and then append their body

@return string