src/Transaction/TransactionBuilder.php
public function signWith(SigningInterface $signer)
    {
        $decoratedSignature = $signer->signTransaction($this);

        $this->signatures[] = $decoratedSignature;

        return $decoratedSignature;
    }
public function signWith(SigningInterface $signer)
    {
        $decoratedSignature = $signer->signTransaction($this);

        $this->signatures[] = $decoratedSignature;

        return $decoratedSignature;
    }
Uses $signer to add a new DecoratedSignature to this TransactionBuilder

@param SigningInterface $signer
@return DecoratedSignature