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

i'm using sublime text 3 sublimelinter-php plugin, uses phplint. if supply simple bit of code, says "error: guessed signature of function foo() void()":

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

what mean? googling turns references error, can't find explanations or instructions on fixing it.

should change code avoid error, or should ignore it?