exit() with message and non-zero exit status

Question

I have two files: first.php:

#!/usr/bin/php
<?php
 exit("Unable"); //1
 #exit(1);  //2
 #exit(); //or exit(0) //3
?>

second.php:

#!/usr/bin/php
<?php
 exec("./first.php",$out,$err);
 var_dump($out);
 echo "\n".$err;
?>

Now, When I run second.php with line #1 in first.php I have "Unable" in $out and 0 in $err. but with two other exits I have that digit in $err.
How can I have non-zero in $err when I execute exit with string message?
I have tested 2>&1 but it's useless.

Answer 1

exit("hi");

Is the same as:

echo "hi";
exit(0);

So just echo the statement :)

echo "Unable";
exit(2);