Although the solution of Halayem Anis is very creative, I think its important to note that you can never be sure that a PHP script keeps running in the background. So if you choose to start your script on "Apache start", then you probably end op resetting Apache quite often, simple to reboot your script.
I assume that's even how you came to this question, as on a normal server you never have to touch the Apache reset button. It starts on system start and then it just runs. If that was the case, you could simple run your php myscript.php command on start up.
Considering there is no way to make sure the script keeps running, I would use a different approach, where I check if it is running and if not, restart it.
So the first step is to make it possible to track if the script is running. I would go for the simple approach where your myscript.php writes a single byte to a file every 5seconds or so. This way I can use the last modified time on the file to see if it is still running, because last modified time + 5 seconds < now == not running.
You could also store the last access time in a database every 5 seconds or so. Might be slightly faster then accessing files if you have a lot of traffic.
The second part is to have each request check if the script is running. For this two work I would use the PHP.ini to prepend a php script on every request. You can do it with the auto_append_file option.
This prepend script would work like this:
<?php
$filename = 'checkonline.txt';
$cmd = "php myscript.php";
if (filemtime($filename)+5<time()) {
//run in background without freezing php
//based on code posted on PHP exec manual, linked below
if (substr(php_uname(), 0, 7) == "Windows"){
pclose(popen("start /B ". $cmd, "r"));
}
else {
exec($cmd . " > /dev/null &");
}
}
?>
Make sure to check how filemtime and exec work and what you need to keep in mind. They work slightly different on Windows/*nix.
