Basically what you need to do is read your the_content data before it is sent to your visitors browser and then do a string replace based on key words that you define.
The following example should be placed in your functions.php file.
to add more keywords place a coma at the end and start a new line.
I suggest you don’t go crazy with the number of links you create.
As an option the add_filter is provided for both your the_content and the_excerpt blocks but you should probably limit this tool to only the_content so it shows on single pages. That is up to you.
$replace = array(
// ‘Find This’ => ‘Replace It With This’
‘chicken’ => ‘<a href="http://www.bigjimmystasteychicken.com/link">chicken</a>’,
‘hamburgers’ => ‘<a href="http://www.mikesloveburgers.com/link">hamburgers</a>’
$text = str_replace(array_keys($replace), $replace, $text);
add_filter(‘the_content’, ‘my_textlink_ads’); // to replace in the_content
add_filter(‘the_excerpt’, ‘my_textlink_ads’); // to replace in the_excerpt
Now you can create your own textlink ads without the need for a external service.