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    I made some mistakes, it works now.
It's pretty simple, I'm ashamed to bother the XOOPS community with my questions 

Here is the thing, hopefully it will be usefull to someone :
File /xoops/html/logusernav.php :
 $ip_to_log = $_SERVER['REMOTE_ADDR']; 
$url_to_log = $_SERVER['REQUEST_URI']; 
if ( !$xoopsUser ) { 
    $uid_to_log = "0"; 
    $uname_to_log = "ANONYMOUS_NONAME"; 
} 
else { 
    $uid_to_log = $xoopsUser->getVar('uid'); 
    $uname_to_log = $xoopsUser->getVar('uname'); 
} 
 
$content_to_log = date('Ymd-H:i:s')." ".$ip_to_log." ".$uid_to_log." ".$uname_to_log." ".$url_to_log."n"; 
 
$log_file_name = XOOPS_ROOT_PATH.'/logusernav.log'; 
 
if (!$log_file_handle = fopen($log_file_name, 'a')) { 
    echo ""; 
} 
else { 
    if (fwrite($log_file_handle, $content_to_log) === FALSE) { 
        echo ""; 
    } 
    @fclose($log_file_handle); 
} 
?>  
Code to add at the end of /xoops/html/footer.php :
 include(XOOPS_ROOT_PATH."/logusernav.php");  
It gives you the following logs :
 20070110-16:27:46 192.168.1.124 0 ANONYMOUS_NONAME /xoops/html/index.php 
20070110-16:28:27 192.168.1.124 5 jcoiron /xoops/html/index.php 
20070110-16:29:16 192.168.1.124 5 jcoiron /xoops/html/ 
20070110-16:29:18 192.168.1.124 5 jcoiron /xoops/html/modules/istats/ 
20070110-16:36:45 192.168.1.124 5 jcoiron /xoops/html/ 
20070110-17:42:02 192.168.1.124 5 jcoiron /xoops/html/modules/news/ 
20070110-17:42:05 192.168.1.124 5 jcoiron /xoops/html/modules/news/article.php?storyid=1279 
20070110-17:45:38 192.168.1.124 5 jcoiron /xoops/html/modules/xoopsmembers/  
Warning ! This produce 
a lot of logs.
If you don't need to keep a record of everything your users do, the module UserVisit is probably what you're looking for :
http://sourceforge.net/project/showfiles.php?group_id=41586&package_id=105497&release_id=207281