PHP无法运行Python函数子进程

I've a good python script that can run on command line (it use some external program in subprocess call). But the problem is that when I try to run it from PHP by system("python test.py") it seems stuck at this line: warp is simple command line example "gdalwarp input.hdf output.tif"

**output = subprocess.check_output(warp, shell=True);**

Please note that I can run it from python script but cannot run from PHP script (it seems cannot run subporcess and os.path.exists

Thanks