Executing R script from PHP

index.php:

<?php
    echo exec("Rscript foo.R");
?>

foo.R:

#!/usr/bin/env Rscript
print("Before!")
#library(rJava);
print("After!");

Output on webpage:

[1] "After!"

Now this is an expected output as exec returns the last line from the result of the command. Now since I want to access the mongodb database which needs rJava and RMongo libraries I modified the above code a little bit.

foo.R:

#!/usr/bin/env Rscript
print("Before!")
library(rJava);
print("After!");

Output on webpage:

[1] "Before!"

Now I don't understand this output. I expected the same output as before i.e [1] "After!". It's as if the R code starting from the library import line doesn't exist at all. I have tested the above code (and the omitted one where I use kmeans on the data grabbed from mongodb) in R shell and it works as expected.

What's wrong with importing a library in a R script which is meant to be executed from PHP?

Update 1:
Interestingly, the modified foo.R gets invoked and executed as per my expectation if I invoke index.php from command line.

$ php index.php 
Loading required package: methods
[1] "After!"

So my conclusion is that my normal user account can execute index.php which in turn can load libraries in foo.R but it seems www-data user account has no permissions to load libraries in R.

So now the question is how do I give permissions to www-data to load R libraries?

Update 2:
I solved this issue temporarily by changing apache user from www-data to my current user but I'm aware this is a huge security risk and I'm looking for an alternative solution.