Recommendations on image verification libraries?

Hi,

I'm using PHP 4.4.4. I want to block automatic submissions from
spammers on our web site, so I'd like to use some PHP libraries that
generate those image graphics and require the user to type the
characters in the graphics. Do you have any recommendations on things
you have used that are easy to implement and effective?

Thanks for any advice or experiences you've had, - Dave

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