If you are annoyed by the behavior of isset() concerning null values, here is a handy function for you. its similar to array_key_exists but, its a lot more flexible and can check for multiple array keys across multiple arrays.
Not recursive!
Tested on php 8.1.6, linux
<?php
/**
* is_set
* @author DJ Eckoson
* @link @eckosongh Facebook Page
* checks whether variable names are set within the global space or they exists as an key and return if they are set (even if their values are null)
* @param string $var_name
* name of the first variable to check
* @param array|null|string
* optional array to check for key (if null, checks from $GLOBALS) OR
* other variable names to check OR
* other variable names and their associated arrays to their right (use null for global variables, optional if its the last parameter)
* check examples below
*/
function is_set(string $var_name, array|null|string ... $args): bool {
 $vars[$var_name] = null;
 if (array_key_exists(0, $args)) {
 if (is_array($args[0])) {
 $vars[$var_name] = $args[0];
 } elseif (is_string($args[0])) {
 goto main;
 }
 unset($args[0]);
 }
 main:
 if ($args) {
 $args = array_reverse($args);
 $cur_array = null;
 array_walk($args, function ($value) use (&$cur_array, &$vars): void {
 if (!is_string($value)) {
 $cur_array = $value;
 } else {
 $vars[$value] = $cur_array;
 }
 });
 }
 foreach ($vars as $name => $array) {
 if (!array_key_exists($name, $array??$GLOBALS)) return false;
 }
 return true;
}
// Examples
$arr1 = range(0, 5);
$arr2 = [
 'a' => 1,
 'b' => 2,
 'c' => 'hELLO wORLD!'
];
$gender = 'male';
$age = 12;
var_dump(is_set('age')); // true
var_dump(is_set('age', null)); // true
var_dump(is_set('age', $arr1)); // false
var_dump(is_set('age', array())); // false
var_dump(is_set('age', array('age' => 48))); // true
var_dump(is_set('age', 'arr1', null, 'b', $arr2, 0, 3, 4, $arr1, 'gender')); // true
var_dump(is_set('age', 'arr1', null, 'b', $arr2, 0, 3, 4, $arr1, 'gender', null)); // true
$c=$d=$e=$a=2;
$arr = [1,4];
var_dump(is_set('a', 'c', null, 0, 1, $arr)); // true
var_dump(is_set('a', 'c', null, 0, 4, $arr)); // false
?>
 Note:
 it won't work for variables declared locally inside a function;
 however you can use it for checking if array keys exists inside a function