<?php
//Set our time variable at the start of the script
$start = time();

//Set the counter to run
$runCounter = TRUE;

//Start our loop
while (1) {
    if ($runCounter = TRUE) {
        //Calculate how many seconds after start
        $counter = time() - $start;
    }

    if ($counter > 29 && $runCounter == TRUE) {
        //Stop the counter running as we no longer need it
        $runCounter = FALSE;

        //Run your action here, it will execute after 29 seconds (on the 30th second)
    }
}
?>