custom captcha image wordpress form - php

Question

I want to create a captcha image for my WordPress ajax singup form. for this reason, I create a captcha.php file in root for generating images that worked successfully.
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;
}
$image = imagecreatetruecolor(130, 40);
imageantialias($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__).'/wp-content/themes/theme_name/fonts/ttf/dana-fanum-black.ttf'];
$string_length = 5;
$captcha_string = generate_string($permitted_chars, $string_length);
$_SESSION['captcha_text'] = $captcha_string;
for($i = 0; $i < $string_length; $i++) {
$letter_space = 120/$string_length;
$initial = 15;
imagettftext($image, 24, rand(-15, 15), $initial + $i*$letter_space-10, rand(25, 45), $textcolors[rand(0, 1)], $fonts[array_rand($fonts)], $captcha_string[$i]);
}
header('Content-type: image/png');
imagepng($image);
imagedestroy($image);
And in my form show an image and text field.
<img data-no-lazy="1" src="/captcha.php" alt="CAPTCHA" class="captcha-image">
<input type="text" id="captcha" name="captcha_challenge" pattern="[A-Za-z]{5}" required="" placeholder="">
when i get $_SESSION['captcha_text'] in my wordpress ajax function this have null value.
session_start();
var_dump($_SESSION['captcha_text']);
/* return null */