是这样的:

function makeNonNestedRecursive(array &$out, $key, array $in){
    foreach($in as $k=>$v){
        if(is_array($v)){
            makeNonNestedRecursive($out, $key . $k . '_', $v);
        }else{
            $out[$key . $k] = $v;
        }
    }
}

function makeNonNested(array $in){
    $out = array();
    makeNonNestedRecursive($out, '', $in);
    return $out;
}

// Example
$fooCompressed = makeNonNested($foo);