PHP: $_POST["startId$i"]) != "" - php

Question

I am trying to process a form that is dynamically created and therefore varies in length. The while loop seems to work fine. However, the 'if' statement is not; it should only print the startId$i and corId$i if and only if the form's particular text field was filled in. The code is printing a line for every text field on the form, regardless of if it was left empty or not.
$i = 0;
while(!is_null($_POST["startId$i"])){
if(($_POST["startId$i"]) != ""){
echo "startId: " . $_POST["startId$i"] . " ---<br>";
echo "corId: " . $_POST["corId$i"] . " ---<br>";
}
$i++;
}

Answer 1

$i = 0;
while(isset($_POST["startId$i"])){
if( !empty($_POST["startId$i"]) ){
echo "startId: " . $_POST["startId$i"] . " ---<br>";
echo "corId: " . $_POST["corId$i"] . " ---<br>";
}
$i++;
}
Can you manage with fields names ?
If yes, better way is to name inputs with name="startId[0]" and name="corId[0]" and so on...
Then in PHP you just do:
$startIds = $_POST['startId'];
$corIds = $_POST['corId'];
foreach ( $startIds as $k => $startId ) {
if ( !empty($startId) ) {
$corId = $corIds[$k];
echo "startId: " . $startId . " ---<br>";
echo "corId: " . $corId . " ---<br>";
}
}

Answer 2

You should use empty() in this case:
if(!empty($_POST["startId$i"])) {
...
}

Answer 3

I suggest to check the real content of $_POST. You can do that via var_dump($_POST);
You may find out, for example, that the empty fields contain whitespaces. In that case the trim() function may help.
For example:
while(isset($_POST["startId$i"])){
if(trim($_POST["startId$i"])){
echo "startId: " . $_POST["startId$i"] . " ---<br>";
echo "corId: " . $_POST["corId$i"] . " ---<br>";
}
$i++;
}