array_merge.php

<?php

function cm(array $ar1, array $ar2, int $rc = 0): array
{
    $duplicatePositions = [];
    $merged = [];
    
    foreach($ar1 as $key => &$value){
        if($dk = array_search($value, $ar2, true)){
            $duplicatePositions[$dk] = $ar2[$dk];
        }
        
        if(is_array($value)){
            $value = cm($value, $ar2, $rc++);
        }
        
        $merged[$key] = $value;
    }
    
    foreach($ar2 as $key => $value){
        if(array_search($key, $duplicatePositions)){
            continue;
        }
        
        $merged[$key] = $value;
    }
    
    $rc--;
    
    return $merged;
}

$ar1 = array("color" => array("red", "green"));
$ar2 = array("color" => array("green", "blue"));

print_r(array_merge($ar1,$ar2));
print_r(cm($ar1,$ar2));