command line - Execute php using exec() not working

I am trying to execute a shell command from php command line in following way

php -r '$test = exec("aws s3 cp s3://test/my-container/testing.txt /var/www/files-test"); echo $test;'
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this works and as result files get downloaded from s3 to mentioned destination/var/www/files-test

But when I execute same command from web app it does not work. Code is

$test = exec("aws s3 cp s3://test/my-container/testing.txt /var/www/files-test");
print_r($test);
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it does not work and as output I got

Completed 1 part(s) with ... file(s) remaining

I have ensured apache user has required privileges. What can be missing here?