I would suggest trying one of the PAAS services with PHP support as that may be all you need. You'll have to do some research, but a few of your options are:
Most of the PAAS companies offer a low-end option for testing that may be all you need.
If you need more control and you're familiar with the command line at all I would suggest just getting a cheap Rackspace Cloud Server, Amazon EC2 instance, or similar VPS to run your script on. You can remove the server when you aren't using it so you only pay for the hours that it's on, but even if you leave it on the lowest level Cloud Server is only ~$16 a month.