CAPTCHA for simple PHP Form Handler

Hello, We have a live PHPFormMail? form processor that properly sends reservation requests to one of our email accounts. 1. We need for someone to encode a 3-4 digit alphanumeric CAPTCHA code on the HTML page that requires the submitter to enter a correct code. 2.? We would prefer the CAPTCHA display to be black letters/numbers with a white background. 3. The hosting enviornment is Linux, PHP, MySQL latest and the coder will be given FTP access to the current working form in a live testing enviornment .