E-mail change

What is the bug here? The fgets() fails because the "file" is not readable. Since PHP 7.4, file system read/write failures result in a notice and false return value (this is noted in the upgrading notes).

But is_readable("/sys/class/net/lo/speed") returns true

Try:
<?php
$fname = "/sys/class/net/lo/speed";
$file = fopen($fname, "r");
if (is_readable($fname)) {
    echo ">".fgets($file, 2048)."<";
} else {
    echo "File is not readable";
}
fclose($file);

is_readable() tells you whether you have "r" permissions on the path (which you clearly do, as the fopen is successful). It does not tell you whether actually reading from the FD will be successful. (A simple example is a directory -- on Linux these will fopen fine, but fail reading with errno 21.)

The behavior of /sys files is determined by the kernel and/or device drivers. This particular file happens to reject reads with errno 22.

That's just how things are. If you try to "cat /sys/class/net/lo/speed" you'll get exactly the same result.

Everything clear, thank you for the explanation.