PHP array_merge result incorrectly on new line - php

Question

I am trying to array_merge id’s from one array into another, it’s merging but putting the new values on a new array line:
IDs to be added: $attachment_ids
array (size=2)
0 => string '2620' (length=4)
1 => string '2621' (length=4)
IDs which already exist and will be added to: $existing_data
array (size=1)
0 => string '2589,2561,2432,2422' (length=19)
result of my array_merge:
$merged_data = array_merge($attachment_ids, $existing_data);
is
array (size=3)
0 => string '2620' (length=4)
1 => string '2621' (length=4)
2 => string '2589,2561,2432,2422' (length=19)
My expected result is:
array (size=1)
0 => string '2620,2621,2589,2561,2432,2422'

Answer 1

If you just need the 1 string with all of the values, you could use implode() with the result you have so far...
$merged_data = [ implode(",", $merged_data) ];

Answer 2

Shortly array merge with array_map
$result=array_map(null,$array1,$array2);

Answer 3

array_merge() requires 2 arrays. The existing data is all in a string. With explode, this data can be converted into an array. With implode() a list is created after array_merge().
$attachment_ids = ['2620','2621'];
$existing_data = ['2589,2561,2432,2422'];
$existing_data = explode(',',$existing_data[0]);
$result = [implode(',',array_merge($attachment_ids,$existing_data))];
//$result: array(1) { [0]=> string(29) "2620,2621,2589,2561,2432,2422" }
Update
Alternatively, strings can be chained manually.
$result = [implode(',',$attachment_ids).','.$existing_data[0]];