In order to keep things nice and clean and in this case, simple, you might be better off using array_merge( )
I personally declare any arrays at the top of my class file, in order to make them globally accessible, only because I tend to keep methods free of array declaration (OCD I guess!)
So for me I have an example that might help you, it's something that works for me when needed to add/merge two arrays together:
protected $array1 = array (
  'basic'   => '1',
  'example' => '2',
  'for'     => '3'  
);
protected $array2 = array(
  'merging'     => '4',
  'two'         => '5',
  'associative' => '6',
  'arrays'      => '7',
  'mate'        => '8'
);
Then within your class file, you can use these arrays or any created arrays and merge whenever you want:
public function ExampleOne() 
{
  $firstArray = $this->array1;
  print_r($firstArray);
  
  $secondArray = $this->array2;
  print_r($secondArray);
  $merged = array_merge($firstArray, $secondArray);
  print_r($merged);
}
Each print_r( ) will give you a print out in the console of the data/created array. This is so you can view for yourself that everything has been created correctly and each key has its associated value (check the PHP man pages for a definitive explanation of print_r( ) ).
So, the first array will/should showcase this:
Array
(
  [basic]   => 1
  [example] => 2
  [for]     => 3
)
The second array will/should showcase this:
Array
(
  [merging]     => 4
  [two]         => 5
  [associative] => 6
  [arrays]      => 7
  [mate]        => 8
)
And the array_merge( ) operation will create the final array, which will/should showcase this:
Array
(
  [basic]       => 1
  [example]     => 2
  [for]         => 3
  [merging]     => 4
  [two]         => 5
  [associative] => 6
  [arrays]      => 7
  [mate]        => 8
)
Of course, you don't always have to place/create your arrays at the top of the class file and when needed you can obviously create arrays within a single function if they are only needed/used within there - what I showcased here was just something I had done recently for a project at work (with the data in these arrays being example data of course!)
     
    
array_replace()not suitable? What is the desired output if keys are shared between the two arrays? Robust minimal reproducible examples are important.