Elzo's idea would be a possibility but keep in mind in that case a click on any link (even it isn't a Ad would be registered).
Two questions:
What do you intend with "without change ads operation"? Can you touch the links in your ads and redirect them? Or no e.g. if it is Google AdSense?
Is it Enough to know if a user clicked on a Ad or do you need to know also on which Ad the user clicked?
Maybe, you can add CLASS-Tags to the advertising links and using Elzo's idea but using the "AdLink", insted of "a".
Something like:
<div>
<a class="AdLink">ads link</a> // without modify ads
<a class="AdLink">ads link</a>
<a class="AdLink">ads link</a>
<a class="AdLink">ads link</a>
</div>
and:
$('AdLink').click( function(){
$.post( '/someUrl.php', { data: userID});
});
This is untested but in my opinion it should work.
If you need to know on which link the user clicked, you can add ID-Tags to the advertising links and using Elzo's idea but using the jQuery Selector based on ID's (#) instead of classes ($).
This woul be something like:
<div>
<a Id="AdLink1">ads link</a> // without modify ads
<a Id="AdLink2">ads link</a>
<a Id="AdLink2">ads link</a>
<a Id="AdLink3">ads link</a>
</div>
and:
$('AdLink').click( function(){
$.post( '/someUrl.php', { data: userID});
});
In this case you need also to foreward the Id which AdLink has been clicked.