Execute command line as a different user

My PHP is running as a user with limited rights. I'd like to execute a command line as a different user. I'm trying to delete a file and the PHP user doesn't have access to do this. I know the username and password for the admin user that has rights to delete a file. Is there a command I can use to make PHP run a delete command as the admin user? If so, how can I do this? Thanks, Matt