How to check not null value in php?

if (!($error == NULL))

Simply do:

if ($error)

One would think that the first is more clear, but it's actually more misleading. Here's why:

$error = null;

if (!($error == NULL)) {
    echo 'not null';
}

This works as expected. However, the next five values will have the same and (to many, unexpected) behavior:

$error = 0;
$error = array();
$error = false;
$error = '';
$error = 0.0;

The second conditional if ($error) makes it more clear that type casting is involved.

If the programmer wanted to require that the value actually be null, he should have used a strict comparison, i.e., if ($error !== null)