APL (Dyalog Classic), 27 bytes
{(⍺⊥⍺|(⍺⊤⍵)-⊢)¨1-1↓4⌽,⍳3 3}
{ } is a function with arguments ⍺ (the dimensions h w) and ⍵ (the index i)
⍳3 3 is a matrix of all 2-digit ternary numbers: 0 0, 0 1, ..., 2 2
, enlists the matrix as a vector
1↓4⌽ removes the centre element 1 1 by rotating 4 to the left (4⌽) and dropping one (1↓)
1- subtracts from 1, giving all 8 neighbour offsets
( )¨ applies the function train in parentheses to each offset
⍺⊤⍵ is the base-⍺ encoding of ⍵ - the coordinates of ⍵ in the matrix
(⍺⊤⍵)-⊢ subtracts the current offset, giving the coordinates of a neighbour
⍺| is mod a to wrap around coordinates and stay within the matrix
⍺⊥ decodes from base ⍺