DP [ i ] [ j ] is going to store monotone numbers of length i + 1 considering j + 1 digits ( 1 , 2 , 3 , . .9 )
$ DP = array ( array ( ) ) ; for ( $ i = 0 ; $ i < $ len ; $ i ++ ) { for ( $ j = 0 ; $ j < $ DP_s ; $ j ++ ) $ DP [ $ i ] [ $ j ] = 0 ; }