Convert English Date (DD/MM/YYYY) to (YYYY-MM-DD) from file

<?php

define("NL", "\n");

$file = file("dates.txt");

foreach($file as $key=>$line)
{
    $line = trim($line);

    if($line)
    {
        $split = explode('/', $line);

        $day = $split[0];
        $month = $split[1];
        $year = $split[2];

        $date = $year . '-' . $month . '-' . $day;
        printHTML($date);
    }
}

function printHTML($string, $tag='pre')
{
    echo '<' . $tag . '>' . $string . '</' . $tag . '>' . NL;
}