如何在PHP中附加到另一个数组中的数组

I have an array that contains 4 elements (sub-arrays).

$orders = array( array(), array(), array(), array() );

Now, I want to tell each sub-array to append items to itself, so I iterate through them like so:

for($i=0; $i<4 ; $i++ ) {
  $orders[$i][] = rand();     // this does not work, unexpected '['
}

What is a better way for accomplishing this?

You had a syntax error:

for($i=0; $i<4 ; $i++ ) {
   $orders[$i][] = rand();     // this does not work, unexpected '['
}

But this might be even better:

foreach($orders as $key => $order){
    $orders[$key][] = rand();
}