It is, but you can also return from a logical main. A value is not required, but it is atypical to return void.
When you return in a script, you are returning control to the caller, halting this script termination (assuming you're running the script main), and return the result. So in php, it is perfectly valid to do this:
$res = include '1.php';
print("Include returned: %d" . PHP_EOL, $res);
It is just uncommon to do so (specifically because you require the return in the script main, which is not common and hence why I recommended the use of logical branches instead).