mergeConfigFrom.php

<?php

// source arrays

$source_1 = [
    'abc' => [0 => 'a'],
    'b' => 'c'
];

$source_2 = [
    'abc' => [1 => 'b', 2 => 'c'],
    'b' => 'b'
];


// test 1 - array_merge

$result_1 = array_merge($source_1, $source_2);

var_dump($result_1);


// test 2 - array_merge_recursive

$result_2 = array_merge_recursive($source_1, $source_2);

var_dump($result_2);


// test 3 - recursive_merge

function recursive_merge($arr1, $arr2)
{
    if(!is_array($arr1) || !is_array($arr2)) return $arr2;

    foreach($arr2 as $key => $val2){

        $val1 = isset($arr1[$key]) ? $arr1[$key] : [];

        $arr1[$key] = recursive_merge($val1, $val2);

    }

    return $arr1;
}

$result_3 = recursive_merge($source_1, $source_2);

var_dump($result_3);

//  Test 1 array_merge (undesirable: missing the 'a' value)
//
//    array (size=2)
//      'abc' => 
//        array (size=2)
//          1 => string 'b' (length=1)
//          2 => string 'c' (length=1)
//      'b' => string 'b' (length=1)
//
//  Test 2 array_merge_recursive (undesirable: 'b' index is now an array)
//
//    array (size=2)
//      'abc' => 
//        array (size=3)
//          0 => string 'a' (length=1)
//          1 => string 'b' (length=1)
//          2 => string 'c' (length=1)
//      'b' => 
//        array (size=2)
//          0 => string 'c' (length=1)
//          1 => string 'b' (length=1)
//
//  Test 3 recursive_merge (desired result)
//
//    array (size=2)
//      'abc' => 
//        array (size=3)
//          0 => string 'a' (length=1)
//          1 => string 'b' (length=1)
//          2 => string 'c' (length=1)
//      'b' => string 'b' (length=1)