需要为PHP Captcha图像使用不同的字体

I've got quite a solid PHP form (in terms of preventing spam) that I've been using for years now however I want to change the font for which the captcha uses because quite often people can't distinguish between the numerical '0' and the uppercase letter 'O' and then they think my form "doesn't work" and I get a complaint. I'm not sure what font it's currently using - see I've got quite a solid PHP form (in terms of pr