PHP system functions can call simple commands, but cannot execute octave scripts?

Problem description:

octave is a GNU open source scientific computing software compatible with matlab syntax. I wrote an octave script, specifying octave as the interpreter at the beginning #!/usr/bin/octave
and executing this script directly from the command line can generate the image I want. That is, use ./octavedemo.m arg1 arg2...

Now I want to use PHP system functions such as shell_exec() to execute this command, but it doesn’t work.
In order to test whether there is a permission problem, I tried to use shell_exec() to execute simple ls -l and other commands, and it was successful:

<code>$test = shell_exec("ls -l");
echo $test;</code>

js can receive and print the results on the console, so it should not be a problem with system function execution permissions.

So I’d like to ask what other situations might cause it?
Generating images is slow, so does it take too long? I have set max_excution_time to 600 in php.ini, which should be enough;
Or is it that the image is too big, unlike simple text, so it cannot be generated in such a large size?

If you can provide some experience, I would be very grateful!

Update (resolved):

Originally, I created a new demonstrate directory in the default html directory of apache, and then placed php and other files in this directory. Then I just entered this directory and performed the chmod 777 operation on all the files in it.
Now try to grant 777 permissions to the demonstrate directory, and then the problem will be solved. It should be the reason why files cannot be generated in this directory without granting read and write permissions.
Facts have proved that you still need to think more clearly about the logic of each step in order not to waste time. Thank you to those who answered!