This is used to append an element to an assigned array. If you append to a string value, it is converted to an array value and then appended to. You can explicitly pass name/value pairs, or associative arrays containing the name/value pairs. If you pass the optional third parameter of true, the value will be merged with the current array instead of appended.
Technical Note: The merge parameter respects array keys, so if you merge two numerically indexed arrays, they may overwrite each other or result in non-sequential keys. This is unlike the array_merge() function of PHP which wipes out numerical keys and renumbers them.