replace_md.php

<?php
if(isset($_POST['txt'])) {
    $value = $_POST['txt'];
}

$newtxt = preg_replace('#\*{2}(.*?)\*{2}#', '<b>$1</b>', $value); //bold **something**
$newtxt = preg_replace('#\_{2}(.*?)\_{2}#', '<i>$1</i>', $newtxt); //italics __something__
$newtxt = preg_replace('#\`{2}(.*?)\`{2}#', '<img src = "$1">', $newtxt); //image ``something.jpg/png``
$newtxt = preg_replace('#\-{2}(.*?)\-{2}#', '<br>', $newtxt); //break line ---- 

echo $newtxt;

echo "<br> Source code: " . htmlspecialchars($newtxt);