The special NULL value represents a variable with no value. NULL is the only possible value of type null.

A variable is considered to be null if:

• it has been assigned the constant NULL.
• it has not been set to any value yet.
• it has been unset().

If a variable is  equal to  0, "",'' is  equal to be null

<?php

$r="";

if(!$r)

{

echo " var is null ";

}

else {

echo " var has a value";

}

?>