PHP Captcha Index

<?php

$salt = "#DqX;£dUx+;_Qn[\"CzA8:#M54R|~.GH_+sG";

if ( $_POST ) {
    if ( hash('sha256', $salt .":".strtoupper($_POST['captcha'])) == $_POST['mcaptcha']) {
        print "correct";
    } else {
        print "incorrect";
    }
} else {
    $capt = substr(md5(time()),0,4);
    $capt .= " " . substr(md5(time()),4,4);
    $mdc = hash('sha256', $salt .":".strtoupper($capt));
    print "<form name=\"captcha\" method=\"post\">
    <img src=\"captcha.php\"> <br>
    <input type=\"hidden\" name=\"mcaptcha\" value=\"$mdc\">
    <input type=\"text\" name=\"captcha\">
    <input type=\"submit\" value=\"Submit\"/>
</form>";
}

?>