In
Arabic it's an ordinary
phoneme, and hence, a letter in the
alphabet, the
hamza.
In French it's one of the ways not to pronounce an h, as in hôtel.
In many regional variants of English it's used to pronounce the t: le'er for letter, etc. It's not a separate phoneme.
In uh-oh (which I think in Dutch is a loan word from English) it is definitely essential to its meaning, but does one word a phoneme make?