通过php执行jar文件

I have a jar file that I am attempting to execute through php using

exec('java -jar DUapi.jar '.$USER.' '.$PWORD);

When I do that however, it does not output a text file to the directory like it's supposed to. If I execute the jar on the command line, however, it works properly. If I print the result code, the result is "Array". Could this be an issue with permissions?