I need to merge these 2 arrays by the key aa => “value“

$  array1 = array(     "aa" => array("RM","PR","LE"),     "bb" => array("100a","200a","300a"),     "cc" => array("1.99b","2.99b","3.99b") ); 

The second array may have extra values per key, notice in this example the “ST” value in the “aa” key.

$  array2 = array(     "aa" => array("RM","PR","ST","LE"),     "dd"  => array("UK","UK","UK","UK"),     "ee" => array("123","456","000","789") ); 

To end up as 1 array like this.

$  final_array = array(     "aa" => array("RM","PR","ST","LE"),     "bb" => array("100a","200a",'',"300a"),     "cc" => array("1.99b","2.99b",'',"3.99b")     "dd"  => array("UK","UK","UK","UK"),     "ee" => array("123","456","000","789"), ); 

With the empty values, they are needed. Needs to work like a MySQL join. Any ideas guys?