WordPress - Really Simple CAPTCHA plugin with custom form

index.php

<?php
// https://br.wordpress.org/plugins/really-simple-captcha/
$captcha_instance = new ReallySimpleCaptcha();
$captcha_instance->bg = array( 255, 255, 255 );
$word = $captcha_instance->generate_random_word();
$prefix = mt_rand();
$img_captcha = $captcha_instance->generate_image( $prefix, $word );

if ( isset( $_POST['send'] ) && $_POST['send'] == 'ok' ) {
	if ( $captcha_instance->check( $_POST['prefix'], $_POST['captcha'] ) ) {
		// Congratulations you are human !
	} else {
		// Sorry robot, you can not pass
	}
}
?>

<form method="post">
	<img src="<?php echo home_url().'/wp-content/plugins/really-simple-captcha/tmp/'.$img_captcha ?>" alt="">
	<input type="text" name="captcha">
	<input type="hidden" name="prefix" value="<?php echo $prefix ?>">
	<input type="hidden" name="send" value="ok">
	<button>Send</button>
</form>