What Is This Houseplant? (Philodendron)

I bought this little plant at my local garden centre and the price label just says "green plant". It's a cutting of a much thicker branch which has also been cut off at the top and sealed. Its leaves are long and fairly thin, and vary between different shades of green and a deep purple. Some of the green leaves also have a little yellow spot on them, which I hope are not a disease of some kind.

I'd be so grateful if anyone has any idea what this is so I can work out how to care for it, or if anyone can point me in the right direction.

Thanks!