For nearly 10 years I was using a PHP application that converted words and phrases to affiliate links on my SMF forum and it worked really well to boost my affiliate revenue, especially Amazon affiliate revenue.
About a year ago the script stopped working because of changes with MYSQL and the developers had abandoned the script so I couldn't use it anymore. After a year-long search, I'm happy to say that I found a solution that is working for me and is actually much easier to use than my previous script. It's called CouchMoney (https://couchmoney.net) and after testing it for a few weeks, it's working like a charm.
Though, if you decide to use it, make sure you install the </head> and </body> custom mod (https://www.simplemachines.org/community/index.php?topic=394952.0) to make it easier to install on your site. If you try to place it directly in the index.template file then you'll need to modify the javascript to work.
If you do want to paste it directly into your template file, use this code:
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.0/jquery.min.js"></script>
<script>
$.ajax({
url: "https://couchmoney.net/api/1.1/obj/script?constraints=%5B%7B%20%22key%22%3A%20%22websiteid%22%2C%22constraint_type%22%3A%20%22equals%22%2C%22value%22%3A%20%22[Your Website ID]%22%7D%2C%7B%22key%22%3A+%22valid%22%2C%22constraint_type%22%3A+%22equals%22%2C%22value%22%3A+%22yes%22%7D%5D",
success: function(data){
$str = \'\';
$str = (data[\'response\'].results[0][\'script\']);
$str = $str.replace("\n", "", "g");
var F=new Function ($str);
return(F());
}
});
</script>