(PHP 4 >= 4.0.7, PHP 5, PHP 7)
array_key_exists — Checks if the given key or index exists in the array
   array_key_exists() returns TRUE if the
   given key is set in the array.
   key can be any value possible
   for an array index.
  
keyValue to check.
arrayAn array with keys to check.
   Returns TRUE on success or FALSE on failure.
  
Example #1 array_key_exists() example
<?php
$search_array = array('first' => 1, 'second' => 4);
if (array_key_exists('first', $search_array)) {
    echo "The 'first' element is in the array";
}
?>
Example #2 array_key_exists() vs isset()
    isset() does not return TRUE for array keys
    that correspond to a NULL value, while
    array_key_exists() does.
   
<?php
$search_array = array('first' => null, 'second' => 4);
// returns false
isset($search_array['first']);
// returns true
array_key_exists('first', $search_array);
?>
Note:
For backward compatibility reasons, array_key_exists() will also return
TRUEifkeyis a property defined within an object given asarray. This behaviour should not be relied upon, and care should be taken to ensure thatarrayis an array.To check whether a property exists in an object, use property_exists().