1. Copy kode berikut (kodingan untuk membuat captcha) dan simpan dalam file captcha.php

<?php

session_start();

$permitted_chars = 'ABCDEFGHJKLMNPQRSTUVWXYZ';

function generate_string($input, $strength = 10)
{
    $input_length = strlen($input);
    $random_string = '';
    for ($i = 0; $i < $strength; $i++) {
        $random_character = $input[mt_rand(0, $input_length - 1)];
        $random_string .= $random_character;
    }

    return $random_string;
}

$image = imagecreatetruecolor(200, 50);

imageantialias($image, true);

$colors = [];

$red = rand(125, 175);
$green = rand(125, 175);
$blue = rand(125, 175);

for ($i = 0; $i < 5; $i++) {
    $colors[] = imagecolorallocate($image, $red - 20 * $i, $green - 20 * $i, $blue - 20 * $i);
}

imagefill($image, 0, 0, $colors[0]);

for ($i = 0; $i < 10; $i++) {
    imagesetthickness($image, rand(2, 10));
    $line_color = $colors[rand(1, 4)];
    imagerectangle($image, rand(-10, 190), rand(-10, 10), rand(-10, 190), rand(40, 60), $line_color);
}

$black = imagecolorallocate($image, 0, 0, 0);
$white = imagecolorallocate($image, 255, 255, 255);
$textcolors = [$black, $white];

$fonts = [dirname(__FILE__) . '\fonts\Acme.ttf', dirname(__FILE__) . '\fonts\Ubuntu.ttf', dirname(__FILE__) . '\fonts\Merriweather.ttf', dirname(__FILE__) . '\fonts\PlayfairDisplay.ttf'];

$string_length = 6;
$captcha_string = generate_string($permitted_chars, $string_length);

$_SESSION['captcha_text'] = $captcha_string;

for ($i = 0; $i < $string_length; $i++) {
    $letter_space = 170 / $string_length;
    $initial = 15;

    imagettftext($image, 24, rand(-15, 15), $initial + $i * $letter_space, rand(25, 45), $textcolors[rand(0, 1)], $fonts[array_rand($fonts)], $captcha_string[$i]);
}

header('Content-type: image/png');
imagepng($image);
imagedestroy($image);

2. Ini kodingan untuk di form HTML (silakan customize sesuai kebutuhan)

<form action="" method="post" id="imeiForm">
        <label>Cari :</label>
        <input type="text" name="keyword" id="keyword">
        <img src="captcha.php" alt="CAPTCHA" class="captcha-image"><i class="fas fa-redo refresh-captcha"></i>
        <input type="text" id="captcha" name="captcha_challenge" pattern="[A-Z]{6}">
        </div>
        <button type="submit" name="cari">Cari</button>
    </form>

3. Ini kodingan untuk action ketika form diinput

if ($_POST) {
        $keyword = "";
        if (isset($_POST['captcha_challenge']) && $_POST['captcha_challenge'] == $_SESSION['captcha_text']) {
            // INI ACTION KETIKA CAPTCHA BENAR
            if (isset($_POST["cari"])) {
                $imei = cari($_POST["keyword"]);
                if ($imei[0]['imei'] != NULL) {
                    $imei = 'Imei Terdaftar di database Kemenprin.';
                } else {
                    $imei = "Imei Tidak di database Kemenprin.";
                }
                echo "<script>document.getElementById('keyword').value = '" . $_POST["keyword"] . "'</script>";
                echo $imei;
            }
        } else {
            echo "<script>document.getElementById('keyword').value = '" . $_POST["keyword"] . "'</script>";
            echo '<p>You entered an incorrect Captcha.</p>';
        }
    }

4. Jangan lupa tambahkan session

session_start();

5. Juga tambahkan refresh button di captcha

<script>
        var refreshButton = document.querySelector(".refresh-captcha");
        refreshButton.onclick = function() {
            document.querySelector(".captcha-image").src = 'captcha.php?' + Date.now();
        }
    </script>

6. Juga tambahkan file-file font yang dibutuhkan didalam folder fonts.

7. Selesai.