PHP5.0.5 bug

This might be a bug. I want to try this stiation using 5.1.2 but PDFlib runs only 5.1.x environment.

<?php
$num = 100;

// Doesn't work
header("Content-Length: $num");

// Works
header("Content-Length: ".$num);

// Works
print("test $num");

// Works
print("test ".$num);

?>

I spent 1 hour to find this tricky spec.