PHP - shell_exec似乎有时间限制

I'm running a python script while running a php script online. Because of debugging I made one php script:

<?php
echo shell_exec("python ../reader.py");

If run it (on linux) with php test.php it works and outputs some data. But if I try to access the output on the web, it will give an empty string back.

reader.py takes 8 seconds (so no time limit should be reached).
reader.py:

#!/usr/bin/env python
import sys
import Adafruit_DHT

sensor = Adafruit_DHT.DHT11;
pin = 4;

humiture, temperature = Adafruit_DHT.read_retry(sensor, pin)

if humiture is not None and temperature is not None:
    print("{\"temperature\": " + str(temperature) + ", \"humiture\":"  + str(humiture) + "}")
else:
    print("fail")

Some other commands like ls will work always.

I think there is a problem with the duration but how can I pass this issue?

Solution: Solved my problem giving permissions:

sudo chmod g+s /var/www  
sudo chmod 775 /var/www  
sudo chown -R www-data:www-data /var/www

Chances are that the user your Web server runs as, and whose permissions the script inherits when it is run from a web page, is not allowed to access the hardware, and returns an empty temperature and humidity - or nothing at all.

Another possibility is that the PHP script executes somewhere else than you think, and finds no reader.py script to launch.

Try using "ls ../reader.py" to see whether the script is accessible, and run

"python ../reader.py 2>&1"

to also collect any errors from the python spawning. By the way, is the python executable reachable at all? Have you tried an empty reader2.py that only prints "Hello world"?