$a = array(1=>"a",2=>"b");
$b = array(2=>"e",3=>"c",4=>"d");
$c = array(""=>"num")+$a+ array("2.5"=>"b.c")+$b;

echo "<pre>";
print_r( $a );
print_r( $b );
print_r( $c );
echo "</pre>";

Array
(
    [1] => a
    [2] => b
)
Array
(
    [2] => e
    [3] => c
    [4] => d
)
Array
(
    [] => num
    [1] => a
    [2] => b
    [2.5] => b.c
    [3] => c
    [4] => d
)