Recaptcha - php function

function recaptcha($key) {
 global $captcha_img, $captcha_id, $challenge;
 
 //Curl
 $ch=curl_init();
 curl_setopt($ch, CURLOPT_URL, 'http://api.recaptcha.net/challenge?k='.$key);
 curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
 $response=curl_exec($ch);
 curl_close($ch);
 
 //Parse content
 $regexp='/challenge : '(.*?)',/is';
 preg_match($regexp, $response, $challenge);
 
 //Image captcha
 $captcha_id=$challenge[1];
 $captcha_img='<img src="http://www.google.com/recaptcha/api/image?c='.$captcha_id.'">';
}