Php beginner code doesn't work. Any ideas?

Question

I'm trying to learn php, but when I try to compile this code

<?php

  $n = 15,32;
  $log = log ($n);

  printf ("log (%0.2f) = %.5<br>", $log);

?>

it shows me

Parse error: syntax error, unexpected ','on line 25

but when I try to compile this

<?php

  $n = 15.32;
  $log = log ($n);

  printf ("log (%0.2f) = %.5<br>", $log);

?>

it doesn't print anything. Any ideas why this happens?