php exec function execute php script via browser different from not via browser

I have a PHP script called a.php that calls exec() to execute php command to send data to another platform, and then get data from it. i have found much information. i still failed according to their ways.

a.php uses curl to send data and use json_decode() to parse data from curl_exec() function.

My environment:

• php 5.3.2
  • php.ini
    • safe_mode = Off
    • disable_functions=
• server: apache
• os: linux

in my php class:

I use exec("php a.php > /data0/log/log.log"), then I access it via browser, but i get nothing.

Finally I find that I can use php exec to execute php script, because I find data in the platform where I send data. However a.php shows nothing. i also find nothing in /data0/log/log.log, but if i use php a.php > /data0/log/log.log in linux, then I can find information which i use 'echo, print_r' in a.php, so I think a.php is correct.

I use exec("php -cli a.php > /data0/log/log.log &"), i got undefined function json_decode() in /data0/log/log.log.

I don't know how to fix this problem. But in my another server it can execute successfully. I only use exec("php a.php").

thanks
van