The set_time_limit() function seems to be misbehaving. With the following
script :
<?
set_time_limit(9);
for ($i = 0; $i < 500000; $i++) {
  $table[] = $i;
}     
?>

I get the output :

Content-Type: text/html

<br>
<b>Fatal error</b>:  Maximum execution time of 60 seconds exceeded in <b>test.php</b> on line <b>3</b><br>

So first there is a cosmetic problem with the 60 (which is my php.ini default).
Second, the script seems to take much more time than 9 seconds
before this message is displayed.
Third, AFTER this message is displayed, the script still takes
CPU time for several seconds or even minutes with big
values of time_limit. I suppose it frees its data. Is there something
to do ?