Running Shell Script from PHP Page - php

Question

What I'd like to do:
When mydomain.com/index.php is viewed, run a script, that logs in on another machine (different from the one serving the index.php) in the local net of the server and runs a Python script.
What I have so far:
In the index.php I call
shell_exec('/var/www/dir/save.sh '.$ARG1.' '.$ARG2);
The script save.sh:
#!/usr/bin/expect -f
set cmd "python /path/to/python_script.py"
set file [lindex $argv 0]
set content [lindex $argv 1]
spawn ssh usr#local_ip "$cmd $file $content"
expect "assword:"
send "PWD\r"
interact
I ran sudo chmod www-data+x save.sh to enable the PHP-Site to run the command. When I run sudo -u www-data ./save.sh arg1 arg2 in the terminal, everything works as expected and I can see the output of python_script.py (that I print for testing).
However, when calling index.php and echoing the output of the shell_exec-Call, I see everything up to user#ip's password: but not the output of python_script.py and I can check, that the script has not been invoked on the other machine.
Any idea, what I'm missing?
I'm glad for any help! :)
UPDATE
Thanks for the quick responses. Sorry for not making myself clear enough.
First of all: This is not used for a public/commercial/elaborate website but rather for private fun and mydomain.com is locked by auth_basic.
My setup: Server1 is hosting mydomain.com. Server2 is running some stuff for private entertainment, e.g. smart home software and Telegram bots.
What I want to do: When calling mydomain.com/index.php, I want to save same parameters (actually just True or False) to Server2, to use them for some of the scripts running there. Server1 and Server2 are in the same local network but only Server1 is reachable from the outside, hence the idea to login via SSH and run a Python-Script.
I appreciate any advise on how to do this in a better way or to make my inteded solution work. :)
UPDATE 2:
Thanks symcbean for pointing to errors in the script. I have to admit, that I copy pasted it from other stackoverflow threads.
Thanks also to ADyson for pointing out better ways to handle the issue. I will definetly keep them in mind for bigger projects.
In the meantime I was able to get the script running. It is still very basic, does not track excection of the Python script and does not handle errors. Again, I wan't to point out, that I use this for private usage only and mydomain.com/index.php will only be up for limited time (it will be an andvent calender and will be taken offline after xmas). But just for completeness sake, here is what I came up with:
#!/usr/bin/expect -f
set cmd "python /path/to/python_script.py"
set file [lindex $argv 0]
set content [lindex $argv 1]
spawn ssh usr#local_ip
expect "assword:"
send "PWD\r"
expect "*\$ "
send "$cmd $file $content\r"
expect "*\$*"
send "exit\r"
interact

Answer 1

Leaving aside the peculiar choices before you got to that point, what exactly were you expecting to happen when the expect script gets to
interact
?
There is nothing in your shell script to start the python script, nor to track its execution, nor to exit the shell. Meanwhile you PHP code is blocked until the local script exits.
While there are lots of ways of fixing this, I am hesitant to suggest any without a better understanding of why you chose such an elaborate mechanism to handle the invocation.