How to properly execute exec in php?

Question

PHP

How to write such code correctly so that it is executed and the value is written to the variable?

$per = exec('hddtemp /dev/sda | awk \'{print $4}\' | cut -c 1-2');
echo $per;

I tried to escape slashes, the output is also empty.
If you run this command in the console, the output is similar to this:

[[email protected] ~]# hddtemp /dev/sda | awk '{print $4}' | cut -c 1-2
45

Answer 1

Found the problem. Thanks for the tip Cyril Saksin The problem is that in the console I tried from the root, and the puff is executed from the user. Specified to the user the full path to hddtemp and the problem was solved.
Example:
$result = `/usr/sbin/hddtemp /dev/sdb | awk '{print $4}' | cut -c 1-2`;

Answer 2

$result = `hddtemp /dev/sda | awk '{print $4}' | cut -c 1-2`;