These methods have been shown to work for legacy Philips DVDRs such as the HDRW72x, DVDR98x, 7x, 61x, and recently also the newer 33xx machines.
The electronic procedure:
Switch off the unit at the wall plug. On the front panel, hold in the <RECORD> and <EJECT> buttons at the same time, and keeping them held, switch on the power at the wall again. Keep holding the buttons until the disc tray opens.
The manual procedure:
There should be a small slot on the lower side of the unit near the front and center of the disc assembly. By sliding a small plastic numb from one side to the other unlocks the door so it can maually be opened the rest of the way. You will need a strong, thin object to slide that release nub. A small knife or nail would work. Of course you should follow all safety precaution such as removing power and not damaging anything else.