// Here's our fruity array
$fruits = ['apple', 'pear', 'banana'];

// Use it in an `if` statement
if (array_key_exists("banana", $fruits)) {
 // Do stuff because `banana` exists
}

// Store it for later use
$exists = array_key_exists("peach", $fruits);

<?php
$search_array = array('first' => null, 'second' => 4);

// returns false
isset($search_array['first']);

// returns true
array_key_exists('first', $search_array);
?>