check.php:

<?php
if(isset($_POST['randomString']) && isset($_POST['md5_string'])) {
    $randomString = $_POST['randomString'];
    $md5_string = $_POST['md5_string'];
    if(md5($randomString) == $md5_string) {
        echo "Strings Match";
        } else {
        echo "Something fucked up<br>";
        echo "Received md5: " . md5($randomString) . "<br>";
        echo "POST md5: " . $md5_string;

    }
} else {
echo "POST was improperly set.";
}
?>


index.php:
<?php
    $chars = '123456789abcdefghjklmnopqrstuvwxyzABCDEFGHJKLMNOPRSTUVWXYZ';
    $randomString = '';
    $linemin = 6;
    $linemax = 8;
    $lines = 4000;

for ($x = 0; $x < $lines; $x++) {
    $randline = rand($linemin, $linemax);
    for ($i = 0; $i < $randline; $i++)
    {
        $randomString .= $chars[rand(0, strlen($chars)-1)];
    }
    $randomString .= "\n";
}

$md5_string = md5($randomString);

$size = mb_strlen($randomString, 'latin1');
if($size >= 1024) {
    $size = round($size / 1024, 2).' KB';
    } else {
    $size = $size.' bytes';
}
echo "Size of randomString: " . $size;

?>

<form action='check.php' method='POST'>
<textarea name='randomString' cols=20 rows=20><?php echo $randomString; ?></textarea><br>
<input type='text' size='40' name='md5_string' value='<?php echo $md5_string; ?>'><br>
<input type='submit' name='submit' value='submit'>
</form>