foreach 输出遇到的有关问题，求各位老师解决

$citycode = array("北京"=>"101010100","海淀"=>"101010200","朝阳"=>"101010300");

foreach ($citycode as $arr => $value) {
 $citycode[$arr] .= ',';
}
// saveFile('file.txt',$arr);
file_put_contents('file.txt', $citycode);

?>

$arr = implode(',', $citycode);
saveFile('file.txt',$arr);

$citycode = array("北京"=>"101010100","海淀"=>"101010200","朝阳"=>"101010300");
saveFile('file.txt',implode(',',$citycode));