Run every X seconds

test.php

<?PHP

//-- Run at least every 5 seconds
$diff = 5;

//-- Get my time
$time_check = 0;

//-- Always run
while (1) {
  //-- Get time now
  $time = microtime();
  if ((microtime() - $time_check) > $diff) {
    //-- WORKING CODE
    //-- Reset the time check
    $time_check = microtime();
  }
}