<?php

$dates = [
    '16-01-2017',
    '16-01-2016',
    '01-01-2020',
    '03-06-2009',
];
$xdate = '16-01-2017';

// Ejemplo de uso:
if ($dates[0] === $xdate) {
    $wrongDates = GetWrongDates(array_map('strtotime', $dates));
    foreach ($wrongDates as $i) {
        printf('La fecha %s es menor a la fecha anterior (%s)' . PHP_EOL,
               $dates[$i],
               $dates[$i - 1]);
    }
    exit;
}

echo 'To2 bien.';

// -----------------------------------------------------------------------------

function GetWrongDates(array $dates)
{
    $wrongDates = [];
    for ($i = 1, $n = count($dates); $i < $n; $i++) {
        if ($dates[$i] >= $dates[$i - 1]) {
            // Ok.
            continue;
        }
        $wrongDates[] = $i;
    }

    return $wrongDates;
}