How can I find the maximum and minimum date in an array?

max() and min() works fine with your array:

echo "Latest Date: ". max($dates)."\n";
echo "Earliest Date: ". min($dates)."\n";

please Try this

$date_arr = array('0' => '20-05-2015', '1' => '02-01-2015', '2' => '30-03-2015');
for ($i = 0; $i < count($date_arr); $i++)
{
    if ($i == 0)
    {
        $max_date = date('Y-m-d H:i:s', strtotime($date_arr[$i]));
        $min_date = date('Y-m-d H:i:s', strtotime($date_arr[$i]));
    }
    else if ($i != 0)
    {
        $new_date = date('Y-m-d H:i:s', strtotime($date_arr[$i]));
        if ($new_date > $max_date)
        {
            $max_date = $new_date;
        }
        else if ($new_date < $min_date)
        {
            $min_date = $new_date;
        }
    }
}
echo date('d-m-Y',strtotime($max_date));
echo date('d-m-Y',strtotime($min_date));

<?php

$date_arr=array(0=>'20-05-2015',1=>'02-01-2015',2=>'30-03-2015');

usort($date_arr, function($a, $b) {
    $dateTimestamp1 = strtotime($a);
    $dateTimestamp2 = strtotime($b);

    return $dateTimestamp1 < $dateTimestamp2 ? -1: 1;
});

echo 'Min: ' . $date_arr[0];
echo '<br/>';
echo 'Max: ' . $date_arr[count($date_arr) - 1];


?>