CPU Temperatur with PHP

I want to see the CPU Temperatur on my website. I want to execute a command with PHP in the console. My Problem is that I only get the result 255.

Here is my code:

<?php

exec('vcgencmd measure_temp', $array, $output);
echo $output

?>

Someone know why I only get the result 255?

Thanks

Tjum28

[I use lighttpd with PHP 7.4 -> Webserver]