How can I insert a new item into an array on any position, for example in the middle of array?
You may find this a little more intuitive. It only requires one function call to array_splice
:
$original = array( 'a', 'b', 'c', 'd', 'e' );
$inserted = array( 'x' ); // not necessarily an array, see manual quote
array_splice( $original, 3, 0, $inserted ); // splice in at position 3
// $original is now a b c x d e
If replacement is just one element it is not necessary to put array() around it, unless the element is an array itself, an object or NULL.