如何在php中获取关联数组中的空值键

i am doing validation of html form in php, I want to print out the key of an empty field,

$url = $_POST['url']; $start_time = $_POST['start_time']; $end_time = $_POST['end_time'];

$arr = array("Url"=>"$url","Start Time"=>"$start_time","End Time"=> "$end_time");

Instead of....

$url = $_POST['url']; $start_time = $_POST['start_time']; 
$end_time = $_POST['end_time'];

I'd suggest

$err =0;
 // repeat following block for every mandatory input
if ((isset($_POST['url']) && (!empty($_POST['url']))){
$url = $_POST['url'];
} else {$err++;  $errmsg .='You need to enter a URL<br>';}

...

if ($err > 0){
echo $errmsg;
} else {
// process submitted form 
}

eg, I prefer to look for errors first, before doing anything with a user provided variable.