My work : Sometimes when we run scripts from the browser and suddenly we get white screen without any error. Even i have faced this when i was exporting product from magento store.So after few hours of search i came across above question.
A function registered with register_shutdown_function will be called when the execution time limit is reached. You can test the limit with a busy wait:
echo “shutdown function called\n”;
set_time_limit(1); // Set time limit to 1 second (optional)
for (;;) ; // Busy wait