You get 2 types of array in PHP. One uses an index going from 0,1,2,3... and the other uses a string as the index. The code you supply is using an associative array (using a string to index the array) but you now seem to want to use a numeric index ie -1 (which is outside the allowed range).
A quick and dirty solution would be to use the string "-1" as the index. A better solution might be to think about why you even need a -1 entry in your array. It doesn't make much sense to me but then I'm not writing your code