为什么gdal_calc.py只能在终端中工作，但是使用PHP exec或Python脚本什么都没发生？

Question

I have a simple command using gdal_calc that only works directly in the mac terminal. But, when I run in PHP using exec or in python, nothing happened.

I have a simple code like this :

gdal_calc.py -A input1.tif -B input2.tif --outfile=result.tif --calc="A+B"

In PHP:

exec('gdal_calc.py -A input1.tif -B input2.tif --outfile=result.tif --calc="A+B"', $output, $response);

this doesn't do anything so i run it direclty using python script

#!/usr/bin/env python
import subprocess
subprocess.call('gdal_calc.py -A input1.tif -B input2.tif --outfile=result.tif --calc="A+B"', shell=True);

no error and no result. When i use gdal_grid or gdalwarp it works well in both languages