If you want to add Google's reCaptcha (https://www.google.com/recaptcha/intro/index.html) to your Drupal 7 forms programmatically you need to follow these two steps:
1) Install and enable captcha (https://www.drupal.org/project/captcha) and
recaptcha (https://www.drupal.org/project/recaptcha) modules. The best way to
do this is to use drush:
drush en -y recaptcha
2) Add this snippet to any Drupal form's code you wish to have this captcha on:
$form['captcha'] = array( '#type' => 'captcha', '#captcha_type' => 'recaptcha/reCAPTCHA', );
And you're done! I hope from now on, spam bots will bother you less.
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