PHP - If-then-else And Checking For Null

$sql = "SELECT SUM(IF(`submitdate` IS NULL , 1 , 0 )) as 'Survey Started But Not Completed' FROM `survey_$surveyid`";
$statement = $dbh->prepare($sql);
$statement->execute();
$result = $statement->fetch(PDO::FETCH_OBJ); //pass that data to an object

stdClass Object
(
    [Survey Started But Not Completed] => 
)

$result  = 0;

$result = array(['Survey Started But Not Completed'] => "0");

SELECT IFNULL(trans, 0) AS 'Transactions', customer.cID AS 'Customer ID', customer.surname AS 'Surname'
FROM customer
LEFT JOIN account
ON (account.cID = customer.cID)
WHERE IFNULL(trans, 0);

2 PART QUESTION (so I don't have to ask two questions 😀  )

1.)
My script has tons of PHP variables. Is it better to declare them all at the top of my page like this:
$cat = $words = $day = $lunch = $user = $address = $potato = $_SESSION["goodpeople"] = $dumbquestionList = $b = $hundredMoreVariables = "";
-or-
as needed, like this:
$cat = "";  if($day == 'Thursday') { $cat = 'time_to_feed'; }
$words = "";  $school = array('teachers'=>'boring','lunch'=>'free'); $words = $school['teachers'];
$_SESSION["goodpeople"] = ""; if($_SESSION["goodpeople"] == "" { $_SESSION["goodpeople"] = 'Gandhi'; }

2.)
What are the pros and cons of declaring variables as NULL and using " isset " versus declaring variables as "" and using ==
Example:
$cat = $words = $day = $lunch = $user = $address = NULL;
if( !isset($cat)) { $cat = 'black'; }.........................if(isset($day)) { $sleep = 'none'; }..............etc., etc.
-or-
$cat = $words = $day = $lunch = $user = $address = "";
if($cat == "") { $cat = 'black'; }............................ if($day != "") { $sleep = 'none'; }................etc., etc.

On my particular coding, I can use either the NULL/isset style, or, the ""/== style, with no errors or ill effects.... but something tells me the expert PHP coders prefer one over the other...

Thank you!!

$dataArray = array( 
 
'title' => 'Test Item',
'icon_url' => 'http://url/api/testimage.jpg',
'item_id' => '12345',
'expiration' => 600 
);
 
utf8_encode($dataArray);
 
$responsArray = array( 
 
'success' => '1',
'data' => $dataArray
);
 
 
utf8_encode($responsArray);
 
header('Content-type: application/json');
 
echo print_r($responsArray);
echo json_encode ($responseArray);
echo json_last_error();
 

<?php
session_start();
if(!isset($_SESSION['loopcatch']) || $_SESSION['loopcatch']==null || !is_int($_SESSION['loopcatch'])){
$_SESSION['loopcatch']=0;
}
if($_SESSION['loopCatch'] > 1){
//Email error
die();
}
require 'dbConnect.php';
$categories=array('business', 'music', 'film', 'drum');
//Pull Quotes
//Query to Pull random quote
$mainQuery="SELECT `r1`.`id`, `r1`.`word`, `r1`.`def` FROM `dictionary` AS `r1` JOIN (SELECT (RAND() * (SELECT MAX(`id`) FROM `dictionary`)) AS `id`) AS `r2` WHERE `r1`.`id` >= `r2`.`id` AND `category`=? AND `checked`=0 ORDER BY `r1`.`id` ASC LIMIT 1";
//prepare quotes query
if($prepareQuote=mysqli_prepare($conn, $mainQuery)){
//filter through each category
foreach($categories as $category){
//Bind the variable to the quotes query
mysqli_stmt_bind_param($prepareQuote, "s", $category);
//execute quotes statement
mysqli_stmt_execute($prepareQuote);
//Store quotes result set
mysqli_stmt_store_result($prepareQuote);
//Check how many rows are returned
if(mysqli_stmt_num_rows($prepareQuote) > 0){
//Bind results to variables
mysqli_stmt_bind_result($prepareQuote, $id, $word, $def);
//If $id, $word, or $def is null abort and email error
if(!is_null($id) && is_numeric($id) && !is_null($word) && !is_null($def)){
while($row=mysqli_stmt_fetch($prepareQuote)){
mysqli_autocommit($conn, FALSE);
//Input into second table
$updateQuery="UPDATE `quotes` SET `word`=?, `def`=? WHERE `category`=?";
//prepare insert query
if($updateQuote=mysqli_prepare($conn, $updateQuery)){
//Bind the variables to the insert query
mysqli_stmt_bind_param($updateQuote, "sss", $word, $def, $category);
//execute insert statement
mysqli_stmt_execute($updateQuote);
//Store insert quote result set
mysqli_stmt_store_result($updateQuote);
//Check how many rows are returned on insert quote query
if(mysqli_stmt_affected_rows($updateQuote) > 0){
//If query run sucessfully insert and update; if not rollback.
//mark quote checked
$checkedQuery="UPDATE `dictionary` SET `checked`=1 WHERE `id`=?";
//prepare checked query
if($checkedQuote=mysqli_prepare($conn, $checkedQuery)){
mysqli_stmt_bind_param($checkedQuote, "i", $id);
//execute checked statement
mysqli_stmt_execute($checkedQuote);
//Store checked quote result set
mysqli_stmt_store_result($checkedQuote);
//Check how many rows are returned on checked quote query
if(mysqli_stmt_affected_rows($checkedQuote > 0)){
mysqli_commit($conn);
}
else{
echo 'Error #6 ';
mysqli_rollback($conn);
}
}
else{
echo 'Error #5';
//Email error
die();
}
}
else{
echo 'Error #4';
mysqli_rollback($conn);
}
}
else{
echo 'Error #3';
//Email error
die();
}
}
}
else{
echo 'Error #2';
//Query returned blank result set - Email Error
}
}
else{
//If zero rows returned, uncheck rows in table for that specific category and re-run the query.
$uncheckQuery="UPDATE `dictionary` SET `checked`=0 WHERE `category`=?";
if($uncheckQuotes=mysqli_prepare($conn, $uncheckQuery)){
//Bind the variable to the query
mysqli_stmt_bind_param($uncheckQuotes, "s", $category);
//execute statement
mysqli_stmt_execute($uncheckQuotes);
//Store result set
mysqli_stmt_store_result($uncheckQuotes);
//Check how many rows are returned
if(mysqli_stmt_affected_rows($uncheckQuotes) > 0){
$_SESSION['loopCatch']++;
header("Location: ./pullDailyQuotes.php");
}
else{
//Email error
}
}
}
}
}
else{
//Email error
echo 'Error #1';
die();
}
?>

Is there a better way to have array_intersect_key() return only non-NULL values?  Thanks

    function stripAssociatedArray(array $array, array $keys, bool $removeNull=false):array
    {
        $array = array_intersect_key($array, array_flip($keys));
        return $removeNull?array_filter($array, fn($value) => !is_null($value)):$array;
    }

if ($rowcats->followersrange == "2000-5000")
    {
    $lowrange2000 = "2000";
    $lowestrange = $lowrange2000;
    $numberpeople = $rowcats->numberpeople;
    }
    if ($rowcats->followersrange == "5000-10000")
    {
    $lowrange5000 = "5000";
    if ($lowrange5000 < $lowestrange) { 
    $lowestrange = $lowrange5000; 
    $numberpeople = $rowcats->numberpeople;}
    }

    if ($rowcats->followersrange == "10000-15000")
    {
    $lowrange10000 = "10000";
    if ($lowrange10000 < $lowestrange) { 
    $lowestrange = "$lowrange10000"; 
    $numberpeople = $rowcats->numberpeople;}
    if ($lowestrange == NULL) { echo "yes";}
    echo "$lowestrange";
    }
    if ($rowcats->followersrange == "15000-20000")
    {
    $lowrange15000 = "15000";
    if ($lowrange15000 < $lowestrange) { $lowestrange = $lowrange15000;$numberpeople = $rowcats->numberpeople;}
    }

We are trying to find the lowest range from the entries added to a database, so at the end, we can say the range is FROM... TO...

They might opt for 2-5k, or 5-10k etc.

If they opt for 10-15k, then the lowest range is 10,000.

We have a result of 10000 - 15000.  So the $lowrange10000 = "10000".

Therefore, this should be echoing the $lowestrange from 7th line up in this code, but it remains at NULL, based on:

$lowestrange = NULL;, set at the top of the code.

Why is that?