There's no good way to check weather a key exists in an associative array.

The problem is that if the key exists, but the value is null, is treated the
same as if the key doesn't exist. 

The only way I figure is to check: in_array("key", array_keys($array))
but this seams rather expensive.

(Compare: perl has an "exists" function ...)

Sample:

  $sample = array("a" => "a", "b" => NULL);


  print ":".isset($sample['a'])."<br>\n";
  print ":".isset($sample['b'])."<br>\n";
  print ":".isset($sample['c'])."<br>\n";

  print ":".in_array("a", array_keys($sample))."<br>\n";
  print ":".in_array("b", array_keys($sample))."<br>\n";
  print ":".in_array("c", array_keys($sample))."<br>\n";