array_replace_recursive() replaces the values of
array with the same values from all the following
arrays. If a key from the first array exists in the second array, its value
will be replaced by the value from the second array. If the key exists in the
second array, and not the first, it will be created in the first array.
If a key only exists in the first array, it will be left as is.
If several arrays are passed for replacement, they will be processed
in order, the later array overwriting the previous values.
array_replace_recursive() is recursive : it will recurse into
arrays and apply the same process to the inner value.
When the value in the first array is scalar, it will be replaced
by the value in the second array, may it be scalar or array.
When the value in the first array and the second array
are both arrays, array_replace_recursive() will replace
their respective value recursively.
Note that function will NOT replace a sub-tree from you $array1 if its value in $array2 is an empty array. Even tho the key for this dimension is technically 'set'.
(I suppose it treats it as just another recursive level to dive in, finding no key to compare, backtracking while leaving this sub-tree alone)