Super Simple Captcha script

captcha.php

<?php
/*
*
*Author github.com/Anan5a
*Description Simple Captcha generator script
*/
//require_once './Fnc.php'; //application specific

define('CODESTR','012ABC3456789DEFGHIJKLMNOPQRSTUVWXYZ');

$font = './X51-Outline.ttf';
$hash = $_GET['key'];
if (!is_array($_SESSION['captcha'])) {
    $_SESSION['captcha']=[];
}
if (!array_key_exists($hash, $_SESSION['captcha'])) {
    //assign values

    $str = str_shuffle(CODESTR);
    $code = substr($str, 0, rand(3, 6));
    $validity = time()+360;

    //set values
    $_SESSION['captcha'][$hash] = [
        'code'=>$code,
        'validity'=>$validity,
    ];
    //draw captcha
    $image = imagecreatetruecolor(200, 50);
    $im_color = imagecolorallocate($image, 255, 255, 255);
    imagefilledrectangle($image, 0, 0, 200, 50, $im_color);
    $pixel = imagecolorallocate($image, 0, 0, 255);
    for ($i = 0; $i < 1000 ;$i ++) {
        imagesetpixel($image, rand() %200, rand() %50, $pixel);
    }
    $txt_color = imagecolorallocate($image, 0, 0, 0);
    imagettftext($image, 30, (rand(-5, 5)), 10, 40, $txt_color, $font, $code);
    imagepng($image);
    $im2 = $image;
    imagedestroy($image);
    //print
    echo $image;
}