Running Powershell Script from PHP

I am trying to run a Powershell script from PHP using Apache on Windows Server 2008. I am using the following code to launch the powershell script: 

shell_exec('powershell.exe -command C:\PSscripts\helloworld.ps1')

Unfortunately I cannot get the script to run at all. I keep getting the error message in the apache log file:

File C:\PSscripts\helloworld.ps1 cannot be loaded because the execution of scripts is disabled on this system. 

I have used the "Set-ExecutionPolicy" command to set the policy to Unrestricted and the Apache service is running as Administrator but I can't for the life of me get it to run the Powershell script. 

Running the script directly from Powershell works fine.

Any help would be fantastic! 

Thanks.