Custom captcha in wordpress and inject it to ACF front end form

 First create a file in your theme named: fcaptcha.php
fcaptcha.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]);

$black = imagecolorallocate($image, 0, 0, 0);

$white = imagecolorallocate($image, 255, 255, 255);

$textcolors = [$black, $white];

$string_length = 6;

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

$_SESSION['captcha_text'] = $captcha_string;

imagestring($image, 18, 10, 12, $captcha_string, $textcolors[rand(0, 1)]);

imagepng($image);
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After that, in functions.php file put the below code to start session:

if ( ! session_id() ) {
session_start();
}

And add below lines to functions.php to localize some data:

wp_enqueue_script('jsScript', get_template_directory_uri() . '/js/script.js', array('jquery'), false, true);
$translation_array = array( 'templateUrl' => get_stylesheet_directory_uri() );
wp_localize_script( 'jsScript', 'globalJSObject', $translation_array );

script.js
$('<img/>').attr({ src: globalJSObject.templateUrl+'/fcaptcha.php', id: 'captcha_image'}).appendTo('.solution_captcha');

See the class name of solution_captcha, it can be anything but you will need to put that class in your ACF captcha field.