It's not so clear, but, at least as of PHP 5.3.13, you cannot use this inside of the handler itself as it will throw an error:
<?php
declare(ticks=2);
function tick_handler()
{
unregister_tick_function('tick_handler');
}
register_tick_function('tick_handler');
set_time_limit(0);
usleep(500000);
?>
results in:
warning: unregister_tick_function(): Unable to delete tick function executed at the moment in [filename]
So if you want to unregister it must be done outside of the handler.