PHP Array Exercises : Identify the email addresses which are not unique

<?php
// Define a function to find non-unique values in an array
function array_not_unique($my_array) {
    // Initialize an array to store duplicate values
    $same = array();
    
    // Perform a natural case-insensitive sort on the array
    natcasesort($my_array);
    
    // Reset the array pointer to the beginning
    reset($my_array);

    // Initialize variables to track the previous key and value
    $old_key    = NULL;
    $old_value    = NULL;

    // Loop through the sorted array to find duplicate values
    foreach ($my_array as $key => $value) {
        // Skip NULL values
        if ($value === NULL) { continue; }

        // Check if the current value is the same as the previous value
        if ($old_value == $value) {
            // Store the duplicate values and their corresponding keys
            $same[$old_key]    = $old_value;
            $same[$key]        = $value;
        }

        // Update the variables with the current key and value
        $old_value    = $value;
        $old_key    = $key;
    }

    // Return the array containing non-unique values
    return $same;
}

// Create a test array with duplicate and non-duplicate values
$test_array = array();
$test_array[1]    = 'xyz@example.com';
$test_array[2]    = 'dse@example.com';
$test_array[3]    = 'xyz@example.com';
$test_array[4]    = 'mno@example.com';

// Call the function and display the result
print_r(array_not_unique($test_array));
?>