R help - Jul 2014 - How to bin x,y,z vectors into matrix?

This is probably a basic question, but I haven't been able to Google
anything helpful after trying for days.

I have an R dataframe with x,y,z tuples, where z is a response to x and y
and can be modeled as a surface.

    > head(temp)
             x         y        z
    1 36.55411  965.7779 1644.779
    2 42.36912  978.9721 1643.957
    3 58.34699 1183.7426 1846.123
    4 53.55439 1232.2696 1990.707
    5 50.76167 1115.2049 1281.988
    6 51.37299 1059.9088 1597.028

I would like to create a matrix of mean z values, with rows representing
binned y values and columns representing binned x values, like

                           0<x<40    40<x<60  60<x<80  
x>80
          0<y<800   1000.0    1100.00  1100.00   1000.0
      800<y<1200 1000.0    1200.00  1200.00   1000.0
    1200<y<1400 1000.0    1200.00  1200.00   1000.0
              y<1400 1000.0    1100.00  1100.00   1000.0
thanks
Mark

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