php merge array to the same key code example

Example: array merge with same key in php

Input : $a1=array("a"=>"raj", "b"=>"striver");
        $a2=array("z"=>"geeks", "b"=>"articles");
Output : 
Array
(
    [a] => raj
    [b] => Array
        (
            [0] => striver
            [1] => articles
        )

    [z] => geeks
)