PHP script not executing pdflatex - php

Question

I have a PHP script which accepts data from an incoming POST request, downloads the data to a file, and then runs a shell script. Only some parts of the shells script are executed --- don't know why. The request is
https://knode.work/processLatex.php
where the body is the JSON {data: SOURCE} and SOURCE is LaTeX source text. Here is the PHP script:
<?php
$post_data = $_POST['data'];
if (!empty($post_data)) {
$dir = '/var/www/html/files/';
$file = 'foo';
$filename = $dir.$file.'.tex';
$handle = fopen($filename, "w");
fwrite($handle, $post_data);
fclose($handle);
chdir($dir);
exec("sh texit.sh");
echo("Done!!");
}
?>
And here is the shell script:
/usr/bin/pdflatex foo.tex
/usr/bin/pdfatex foo.tex
rm yuuk.txt
rm *.out *.log *.aux
The /usr/bin/pdflatex lines are not executed, but the other lines are. What the heck? Are not all lines equal?
((Could there be a permissions problem?))
Below is a listing of the ./files directory. I created foo.pdf by running pdflatex foo.tex as root.
-rw-r--r-- 1 root root 207383 Aug 13 17:31 foo.pdf
-rw-r--r-- 1 www-data www-data 11372 Aug 13 18:35 foo.tex
-rw-r--r-- 1 root root 84 Aug 13 17:40 texit.sh
So perhaps the problem lies in the ownership of foo.tex versus that of pdflatex (re comment below) by RiggsFolly. Here is the dope on pdflatex:
lrwxrwxrwx 1 root root 6 Aug 21 2017 /usr/bin/pdflatex -> pdftex
Also, I set everything up as root, e.g.,
drwxr-xr-x 6 root root 4096 Aug 13 18:36 html
What should I do?