GNUPLOT不使用PHP保存输出

I have PHP file, which I use gnuplot to plot sine graph and save the output, but after running the php file in localhost the output does not save anywhere. Here is the code:

<?php

    `echo "set term png;set xrange[-2*pi:2*pi]; 
    set output 'output.png'; plot sin(x)" | gnuplot`;

?>

I use exec() also instead of above code, but no difference:

exec('echo "set term png;set xrange[-2*pi:2*pi]; 
set output \'output.png\'; plot sin(x)" | gnuplot');

However I ran the above code in terminal and the output saved in my home directory. When I run the code using PHP the output should be saved in localhost directory but after running the code it seems nothing happening. I gave different path to save the output (like desktop) but still no output.

Any idea what should I do?