----- 1 ----- 
----- print_aoa(aars) : 
0  Bob   3 Main St.   Mytown   21473  
1  Mary   25 Snyde Way   Snow Hill   32781  
2  Sam   4625 Met St.   Knoxious   75826  
3  John   54 Wilco Ave.   Valcia   45695  
4  Otis   86 Road Way   Bromosburgh   92582  
----- 2 -----
----- Now we remove aars[2] to see what happens in the final sort. 
----- print_aoa(aars)  
0  Bob   3 Main St.   Mytown   21473  
1  Mary   25 Snyde Way   Snow Hill   32781  
3  John   54 Wilco Ave.   Valcia   45695  
4  Otis   86 Road Way   Bromosburgh   92582  
----- 3 - So you can see that the index key values are 0,1,3,4 
----- Now create the a1 array consisting of aars[2] values: 
----- note that new array index values are ordered without skipping [2] 
Array
(
    [0] => Mytown
    [1] => Snow Hill
    [2] => Valcia
    [3] => Bromosburgh
)
----- 4 - For ha ha's here's that array after regular sort(a2) ---- 
Array
(
    [0] => Bromosburgh
    [1] => Mytown
    [2] => Snow Hill
    [3] => Valcia
)
----- 5 - But that's not what array_multisort(a1, aars) wants. 
-----     It wants the a1 array in the original order.
-----     And here is a print of the array now sorted on index[2] 

0  Otis   86 Road Way   Bromosburgh   92582  
1  Bob   3 Main St.   Mytown   21473  
2  Mary   25 Snyde Way   Snow Hill   32781  
3  John   54 Wilco Ave.   Valcia   45695  

OK, so pretty good excercise. A lot of new stuff here.