I believe phase is possible but at the ranges you are talking it is dicey. Vary the period between pulses and look for a point where the received pulse is coincident with the outgoing pulse. If you start with a long period and shorten it until you see the match, the pulse period is 2X the time of flight.
However, light travels around 30 CM per nanosecond. to measure 50 CM (100 CM round trip, 3.333 nS), you would need to be able to generate pulses with a period of better than 3.3nS to some accuracy. Not impossible. You would have to have a pretty sensitive detector to pick up the really short pulses (~1 nS, I'd guess). Typical phototransistors have rise times in the uS range so they are out. Avalanche photo diodes are pretty fast but quite pricey - this one**broken link removed** is $80 from Mouser. I'm sure there are shielding/interference issues.
I'd suggest using ultrasonics if it's a cost sensitive application. Those can meet your needs cheaply.