PHP appends the same elements to a two-dimensional array

$origin_arr = [
    [
		'a'=>'a1',
	    'b'=>'b1',
	    'c'=>'c1',
	],
    [
		'a'=>'a2',
	    'b'=>'b2',
	    'c'=>'c2',
	]
]
$add_arr= [
    'user_source'=>'test'
];
array_walk($origin_arr, function (&$value, $key, $add_arr) {
    
    $value = array_merge($value, $add_arr);
}, $add_arr);
print_r($origin_arr);

//  Print the results 

[
    [
		'a'=>'a1',
	    'b'=>'b1',
	    'c'=>'c1',
    	'user_source'=>'test'
	],
    [
		'a'=>'a2',
	    'b'=>'b2',
	    'c'=>'c2',
    	'user_source'=>'test'
	]
]