This is because wget has a read time limit of 900sec. After it is reached, the download restarts. You can set the timeout higher with the --timeout=seconds or the --read-timeout=seconds argument. Or, you can start it directly from the shell(this way is much better). Here is a link: wget download options Here is the shell code(for Linux):
exec('php yourscript.php > /dev/null 2>&1 &');