Overview
If the destination variable is a set, then assignments can be applied on specific elements, or one element can be appended. If the set is (still) empty, then element 0 can be assigned (a[]{0} = ... ) to append the first element. Going beyond results in errors
Examples
a[] = { 0,1,2,3,4,5 };
a[]{1} = 11; // Replace an element
a[]{6} = 6; // Append an element
echo(a[]);
{0,11,2,3,4,5,6}