array of array

I've to save in an array of array some values.

Problem: I don't know the size of the array before filling it.

I'd like something like

$bigarray = array():

then runnig the code I must create dinamically (I don't know how many) few arrays in the bigarray;
$bigarray["X"] = array():
$bigarray["Y"] = array():

then fill those arrays with few unknow number of values.
array_push($bigarray["X"],$value1);
array_push($bigarray["X"],$value2);

array_push($bigarray["Y"],$value5);
....
but array_push doesn't manage it.

How to do so ?

Bob 


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