i was having a hard problem with /tmp being flooded with sess_ files and them not being deleted by garbage collector. when i set gc_probability to something high (99 or 100) server would return 0 length response for any page with php script. i decided to turn off garbage collector by setting probability to 0. it helped the pages to be displayed correctly.

then i decided to include my own garbage collector script at the end of every page. it would at a given probability rate scan /tmp for sess_ files older than specified number of seconds and delete it. that's when i meet the same problem again.

after shrinking the page to just
===
$dir = opendir("."); //or every other folder
while ($file = readdir($dir)) { 
//print "<br>$file"; 
}
closedir($dir);
===
it still would not return any output. would return zero length response.

shrinking it to 
===
$dir = opendir("/tmp");
echo "<br>".readdir($dir);
echo "<br>".readdir($dir);
closedir($dir);
===

didn't change anything.
shrinking it to
===
$dir = opendir("/tmp");
echo "<br>".readdir($dir);
closedir($dir);
===
finally produced <br>. output.

i guess readdir() failure also brokes garbage collector too.