array_merge.php

<?php

$a['b']=[];
$a['b']['x']="y";
$a['c']=[];
$a['c']['y']="z";
$b=array_merge($a['b'],$a['c']);
print_r($b);
var_dump($b);

/*
Array
(
    [x] => y
    [y] => z
)
array(2) {
  ["x"]=>
  string(1) "y"
  ["y"]=>
  string(1) "z"
}
*/