PHP (Symfony) unit test assertion

I am new to unit testing. So I have one situation that I have encountered.

I have a JSON string and an array that requests transformation

$json = {"values":["Abc", "EFG", "JkL"]};

$transform = [['original'=> 'Abc', 'new' => 'abc'],['original'=>'EFG', 'new'=>'efg']];

and I have a function that accepts these two parameters and makes the transformation of values based on the array.

function transform($json, $arr)
{
  $flat = json_decode($json, true);
  foreach($arr as $ar){
    $key = array_search($ar['original'], $flat);
    if($key)
      $flat[$key] = $ar['new'];
  }
  return json_encode($arr);
}

now the problem I am facing is with the assertion. I am able to do it using multiple assertions in a single test case, but I don't think it's ideal for unit testing.

Any suggestions? also any ideas on how above code can be improved?

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