First problem is that you have a variable with name $d and you are passing
$jd to the function as parameter and the second is that jdmonthname() takes two parameter… Now this is the working code.
<?php
$jd =gregoriantojd(12,02,2010);
echo jdmonthname($jd,1);
?>
solved How to get the full month name using PHP jdmonthname() function?