The unexpected behaviour of isset() for variables which have the value NULL caused a bit of a headache today. One would assume if a variable has any value assigned testing this variable with isset() would return true. This is the case except when the variable has the value NULL assigned. Here a quick example:

$a = 1;
var_dump(isset($a));
$a = null;
var_dump(isset($a));

This outputs:

bool(true)
bool(false)

The PHP documentation for isset() states:

isset() will return FALSE if testing a variable that has been set to NULL.

Kunststube.net has a very good overview on isset() and empty() in combination with NULL .