In PHPLint, what does "guessed signature of the function" mean?

Question

I'm using Sublime Text 3 with the SublimeLinter-php plugin, which uses PHPLint. Even if I supply this very simple bit of code, it says "Error: guessed signature of the function foo() as void()":

<?php
function foo() {
    echo( 'bar' );
}
foo();
?>

What does this mean? Googling turns up references to the error, but I can't find explanations or instructions on fixing it.

Should I change something about my code to avoid this error, or should I ignore it?