captcha.php

<?php
session_start();
$width = 130;
$height = 30;
$font_size = 20;
$font = "./assets/verdana.ttf";
$font = realpath($font);
$chars_length = 4;
$captcha_characters = 'qazwsxedcrfvtgbyhnujmikop0987654321';
$image = imagecreatetruecolor($width, $height);
$bg_color = imagecolorallocate($image, 0, 0, 0);
$font_color = imagecolorallocate($image, 255, 255, 255);
imagefilledrectangle($image, 0, 0, $width, $height, $bg_color);
$vert_line = round($width / 5);
$color = imagecolorallocate($image, 255, 217, 0);
for ($i = 0; $i < $vert_line; $i++) {
    imageline($image, rand(0, $width), rand(0, $height), rand(0, $height), rand(0, $width), $color);
}
$xw = ($width / $chars_length);
$x = 0;
$font_gap = $xw / 2 - $font_size / 2;
$digit = '';
for ($i = 0; $i < $chars_length; $i++) {
    $letter = $captcha_characters[rand(0, strlen($captcha_characters) - 1)];
    $digit .= $letter;
    if ($i == 0) {
        $x = 0;
    } else {
        $x = $xw * $i;
    }
    imagettftext($image, $font_size, rand(-20, 20), $x + $font_gap, rand(27, $height - 5), $font_color, $font, $letter);
}
$_SESSION['captcha'] = strtolower($digit);
header('Content-Type: image/png');
imagepng($image);