It doesn't work now, some corrections needed

[mr-older]

You need to add check for nullable $tx in __construct where $this->txData = $tx.

I.e.:

if(!empty($tx))
{
$this->txData = $tx;
}

[sc0Vu]

Since the $tx is empty array (see: https://github.com/web3p/ethereum-tx/blob/master/src/Transaction.php#L144)

and we do check whether input is hex encoded string.

I don't figure out why we should check nullable $tx in __construct.

[mr-older]

If you are passing array, not hexstring, here: https://github.com/web3p/ethereum-tx/blob/master/src/Transaction.php#L176, your $tx variable doesn't exists after IFs. Your code is not working in that case at all, failing with assignment to nothing. There are numerous methods to solve it, including involving more experienced programmers, of course. Just no-testing-error :)

[sc0Vu]

Oh, I see! I didn't set the default value of $txData.
Good finding, Thanks @mr-older !