This shoul work:
exec("start /B php myscript.php");
Also close all standard streams at the very beginning of myscript.php:
fclose(STDIN);
fclose(STDOUT);
fclose(STDERR);
If you want echo
to still work, you can open a file for writing to stdout after closing the standard streams:
$STDIN = fopen('/dev/null', 'r');
$STDOUT = fopen('myscript.log', 'wb');