PHP executes exec ("Aapt v", $output)

PHP executes exec ("Aapt v", $output) the data is empty ...
But it runs directly under Linux and there's data.
root@ubuntu1204:/usr/local/src# AAPT V
Android Asset Packaging Tool, v0.2
And this folder gives 777 permissions and is capable of executing
EXEC ("pwd", $output) is data-
What's the reason for this?

Reply to discussion (solution)

I also encountered the type situation, you can use the system function, if it is not possible to return the value, it might be the cause of the script executed, Linux has a return value with its configuration environment is related

You run the PHP estimate for Apache ROOT to execute

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