<?php
  function x($a = "foo")
  {
      print $a;
  }
  
  $val = NULL;
  x($val);
?>

This should print out "foo" but it doesnt.

For my understanding this is an inconsequent behaivour. If i have

<?php
  $a = NULL;
  if (isset($a)) print "ok";
?>

"ok" won't be printed. So, if there is no value set, as isset says, there should not be any value set when the value defaults to another value in the functions argument list.